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DOCUMENT RESUME

ED 373 306 CS 011 805

AUTHOR Glynn, Shawn M.TITLE Teaching Science with Analogies: A Strategy for

Teachers and Textbook Authors. Reading ResearchReport No. 15.

INSTITUTION National Reading Research Center, Athens, GA.;National Reading Research Center, College Park,MD.

SPONS AGENCY Office of Educational Research and Improvement (ED),Washington, DC.

PUB DATE 94CONTRACT 117A20007NOTE 34p.

PUB TYPE Viewpoints (Opinion/Position Papers, Essays, etc.)(120) Reports Research/Technical (143)

EDRS PRICE MFOI/PCO2 Plus Postage.DESCRIPTORS Higher Education; *Instructional Effectiveness;

Learning Strategies; Models; Reading Research;*Science Instruction; *Scientific Concepts; SecondaryEducation; Teacher Behavior; *Textbook Evaluation;Textbook Research

IDENTIFIERS *Analogies

ABSTRACTThis paper describes the role of analogy in science

instruction and presents new research on the Teaching-With-Analogiesmodel. After an introductory section, the paper discuses learningscience meaningfully, including constructing relations, strategiesfor learning conceptual relations, the definition of analogy,effectiveness of analogies, and misconceptions caused by analogies.The model described in the paper began with a task analysis of 43middle school, high school, and college textbooks to identify how thetextbook authors used analogies to explain new concepts. The papernotes that the task analysis was supplemented by a study of 10exemplary middlE' school science teachers. The model described in thepaper provides guidelines for constructing analogies systematicallyand using them strategically during science instruction to explainimportant concepts in ways that are meaningful to students. The papershows how exemplary teachers and authors construct effectiveanalogies to help students build on their own relevant knowledge byactivating, transferring, and applying it to new knowledge acquiredfrom textbooks. Contains 44 references, 2 tables of data, and 4figures illustrating aspects of various analogies. (RS)

Reproductions supplied by EDRS are the best that can be madefrom the original document.

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Teaching Science With AnalogiesA Strategy for Teachers and Textbook Authors

Shawn M. GlynnUniversity of Georgia

U S DEPARTMENT OF EDUCATION

EDUCATIONAL RESOURCES INFORMATIONCENTER IERICI

ti This document has been iepioduced asreceived from the person or oiganitationoriginating it

Minor changes have been mode toimprove reproduction ailatity

Points of view or opinions slated in thisdocument do not necesrianty teprosentofficial OEHI Position or policy

NRRC NationalReading ResearchCenter

READING RESEARCH REPORT NO. 15

Spring 1994

BEST COPY AVAILABLE-

NRRCNational Reading Research Center

Teaching Science With Analogy: A Strategy

for Teachers and Textbook Authors


READING RESEARCH REPORT NO. 15Spring 1994

The work reported herein is a National Reading Research Project of the University of Georgia

and University of Maryland. It was supported under the Educational Research and

Development Centers Program (PR/AWARD NO. 117A20007) as administered by the Office

of Educational Research and Improvement, U.S. Department of Education. The findings and

opinions expressed here do not necessarily reflect the position or policies of the National

Reading Research Center, the Office of Educational Research and Improvement, or the U.S.

Department of Education.

NRRC NationalReading ResearchCenter

Executive CommitteeDonna E. Alvermann, Co-DirectorUniversity of Georgia

John T. Guthrie, Co-DirectorUniversity of Maryland College Park

James F. Baumann, Associate DirectorUniversity of Georgia

Patricia S. Koskinen, Associate DirectorUniversity of Maryland College Park

Linda C. DeGroffUniversity of Georgia

John F. O'FlahavanUniversity of Maryland College Park

James V. HoffmanUniversity of Texas at Austin

Cynthia R. HyndUniversity of Georgia

Robert SerpellUniversity of Maryland Baltimore County

Publications Editors

Research Reports and PerspectivesDavid Reinking, Receiving EditorUniversity of Georgia

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VideosShawn M. Glynn, University of Georgia

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About the National Reading Research Center

The National Reading Research Center (NRRC) is

funded by the Office of Educational Research and

Improvement of the U.S. Department of Education to

conduct research on reading and reading instruction.

The NRRC is operate by a consortium of the Universi-

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sensitive to the cognitive, sociocultural, and motiva-

tional factors that affect children's success in reading.

NRRC researchers from a variety of disciplines conduct

studies with teachers and students from widely diverse

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deal with the influence of family and family-school

interactions on the development of literacy; the interac-

tion of sociocultural factors and motivation to read; the

impact of literature-based reading programs on reading

achievement; the effects of reading strategies instruction

on cotnpreiension and critical thinking in literature.science, and history; the influence of innovative group

participation structures on motivation and learning; the

potential of computer technology to enhance literacy;

and the development of methods and standards for

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About the Author

Shawn M. Glynn is Professor of Educational Psy-chology and Science Education at the University ofGeorgia and a principal investigator at the NationalReading Research Center. His research focuseson text-based strategies for learning science con-cepts. He is a Fellow of the American Psycholog-ical Association, and he has served on the editorial

boards of the Journal of Educational Psychology,Educational Psychologist, Educational PsychologyReview, and Contemporary Educational Psycholo-gy. Recently, he was a F,'ulbright Research Fellowin Germany. He may be contacted at the NationalReading Research Center, 318 Aderhold Hall,University of Georgia, Athens, GA 30602-7125.

National Reading Research CenterUniversities of Georgia and MarylandReading Research Report No. 15Spring 1994

Teaching Science With Analogy: A Strategyfor Teachers and Textbook Authors


Abstract. The author describes the role of analo-gy in instruction and presents new research onthe Teaching-With-Analogies Model. This modelis being developed from studies of exemplaryteachers and textbooks; it proviees guidelines forconstructing analogies systematically and usingthem strategically during science instruction toexplain important concepts in ways that aremeaningful to students. The model shows howexemplary teachers and authors construct effec-tive analogies to help students build on their ownrelevant knowledge by activating, transferring,and applying it to new knowledge acquired fromtextbooks.

Many of Galileo Galilei's (1564-1642) col-leagues believed that the earth is standing still,not rotating on its own axis. Galileo could notaccept this view. In an effort to convinceGalileo, his colleagues argued that if the earthwere rotating on its axis, a rock dropped froma tower would not land directly at the foot ofthe tower. They reasoned that if the earth wererotating, the tow would continue to movewhile the rock was falling through the air, withthe result that the rock would hit the ground faraway from the tower's foot. Since this obvi-

1

ously doesn't happen the dropped rockactually lands directly at the tower's foot thecolleagues concluded that the tower and theearth could riot be moving. The colleagues sawno reason to change their belief that the earthand the tower connected to it are standing still.

Galileo countered that the earth, the tower,and a rock held at the top of the tower are allmoving forward with uniform velocity. Whenthe rock is dropped, it gains downward veloci-ty, but it doesn't give up its forward velocity.The downward and forward velocities areindependent. Since the earth, tower, and rockhave the same forward velocity, Galileo ar-gued, the rock will not be left behind by theearth and tower. The rock will land at the footof the tower. Furthermore, he explained, sincethe colleagues share the forward motion of theearth, tower, and rock, they are not in a posi-tion to observe the effects of the earth's move-ment.

To further explain his view to his col-leagues, Galileo drew upon an analogy. Heasked his colleagues to imagine a rock beingdropped from the top of a ship's mast. Galileosaid that, regardless of whether the ship is

2 Shawn M. Glynn

standing still or moving forward with constantspeed in still water, the rock always lands atthe foot of the mast. (The tower in the firstscenario corresponds, of course, to the ship'smast.) The analogy is powerful because itintroduces a new feature: a ship that can beeither standing still or moving. The mast,unlike the tower, is connected to the -hip, abody that the colleagues can view either inmotion or standing still. In either event, thedropped rock lands at the foot of the mast. Bymeans of this powerful analogy, Gaiileo suc-cessfully countered the argument of his col-leagues and advanced his views concerning therotation of the earth and moving frames ofreference.

Analogy in Instruction Today

Just as they did in Galileo's time, scientists stillmake frequent use of analogies. In fact, analo-gies have played an important role in explana-tion, insight, and discovery throughout the his-tory of science (Hesse, 1966; Hoffman, 1980).It is not surprising, therefore, scienceteachers and textbook authors routinely useanalogies to explain complicated concepts tostudents. Often, teachers and authors are un-aware that they are using analogies they doit automatically. Throughout their lessons,especially when responding to students' ques-tions, teachers regularly preface their explana-tions with colloquial expressions such as "It'sjust like ... ," "It's the same as ... ," "It's nodifferent from ... ," and "Think of it as ...." Intextbooks, authors use more formal expressionslike "Similarly," "Likewise," "Along relatedlines," "In comparison to ... ," and "In con-

trast with ...." These expressions are all waysof saying "Let me give you an analogy."

Unfortunately, teachers' and authors'analogies often do more harm than good (Duit,1991; Gilbert, 1989; Thagard, 1992; Treagust,Duit, Jos lin, & Lindauer, 1989). That is be-cause teachers and authors, lacking guidelinesfor using analogies, sometimes use them unsys-tematically, which often causes confusion andmisconceptions in their students. The distinc-tions among a target concept, features of theconcept, examples of the concept, and ananalogy become blurred in students' minds.One solution, of course, would be to adviseteachers and authors not to use analogy. Thatwould be unrealistic because teachers andauthors, like all human beings, are predisposedto think analogically, and, they will use analo-gies, consciously or unconsciously, duringexplanation. The better solutic., is to introduceteachers and authors to a strategy for usinganalogies systematically to explain fundamentalconcepts in ways that are meaningful to stu-dents.

Purpose of Study

This National Reading Research Center reportdescribes the role of analogy in instruction andpresents new research on a model for teachingwith analogies (Glynn, 1991, 1993; Glynn &Hynd, 1994). Task analyses of textbooks andexemplary teachers have been conducted todevelop this model. The purpose of this modelis to provide teachers and textbook authorswith guidelines for constructing analogies andusing them strategically during science instruc-tion. The task analyses examine how exempla-

NATIONAL READING RESEARCH CENTER, READING RESEARCH REPORT NO. 15

iv

Teaching Science With Analogy 3

ry teachers and authors construct effectiveanalogies that help students activate. transfer,and apply relevant existing knowledge whenlearning from textbooks.

LEARNING SCIENCE MEANINGFULLY

Constructing Relations

Meaningful learning is the process of integrat-ing new knowledge with existing knowledge.This process is complex and results from theinteraction of key cognitive processes, such asforming images, organizing, and drawinganalogies (Anderson & Thompson, 1989). Theinteraction of these processes leads to theconstruction of conceptual relations.

Science students should not he viewed ashuman video cameras, passively and automati-cally recording the information transmitted byteachers and textbooks. On the contrary, sci-ence students are active consumers of infor-mation "informavores," if you will who,when learning meaningfully, will challenge theinformation presented to them, struggle with it,and try to make sense of it by integrating itwith what they already know.

One of the questions most often asked byboth new and seasoned science teachers is

"How can I help my students learn conceptsmeaningfully?" The answer is to help studentslearn concepts relationally, not by rote (Glynn,Yeany, & Britton, 1991). That is, studentsshould learn concepts as organized networks ofrelated information, not as lists of facts. Sci-ence teachers often realize this, but are not surehow to facilitate relational learning in their

students, particularly when the number ofstudents in a class is large and the concepts arecomplex. Complex, related concepts are therule rather than the exception in biology (e.g.,photosynthesis and mitosis-meiosis), chemistry(e.g., chemical equilibrium and the periodictable), physics (e.g., gravitational potentialenergy and electromagnetic induction), earthscience (e.g., plate tectonics and precipitation),and space science (e.g., the sun and planetarymotion). In one form or another, many of theseconcepts are introduced to children in theelementary school years; by high school, allstudents are expected to be scientifically liter-ate, that is, to understand these complex con-cepts. So, teachers at all levels play a criticalrole in ensuring that students have a meaning-ful (relational) understanding of fundamentalscience concepts. To be successful in this role,teachers need powerful instructional strategies.

Strategies for LearningConceptual Relations

Studies of experts and novices in fields such asphysics (Chi, Feltovich, & Glaser, 1981) andbiology (Feltovich, 1981) have shown thatexperts are experts, not just because they knowmore facts than novices, but because theirknowledge exists in the form of interrelatednetworks. The relations in and among concep-tual networks are of many kinds, includinghierarchical, exemplifying, attributive, causal,correlational, temporal, additive, and adversa-tive. In an expert, these relations contribute toa high-performance human "information-processing system." This system includes a work-ing-memory component, analogous to a small


4 Shawn M. Glynn

cognitive workbench on which only a fewcognitive operations can be performed at atime, and a long-term memory component,analogous to a set of file cabinets or a comput-er hard disk. Long-term memory provides rawmaterial (knowledge) for working-memoryoperations and stores the products of thoseoperations. The construction of conceptual re-lations enhances an expert s working-memoryand long-term memory performance. Becausethe expert's knowledge is relational, it is easilystored, quickly retrieved, and successfullyapplied. Unfortunately, a student's knowledgeis all-too-often rote, and is hard to apply, diffi-cult to store, and often forgotten.

Teachers and textbook authors need power-ful instructional strategies to help develop a

student's information-processing system intothat of an expert. Concept mapping (Novak,1990) and teaching with analogies (Glynn,1991) are two such strategies. Concept map-ping does facilitate the learning of relationswithin a conceptual network. But what aboutrelationships among different, but in someways similar, conceptual networks? How canteachers and authors develop such relationshipsand use them to bridge conceptual networks?Teaching with analogies can provide answersto these questions.

Need for a Model

The role of analogical thinking in teaching andlearning science has received increasing atten-tion in recent years (Brown, 1993; Clement,1989, 1993; Gentner, 1989; Lawson, 1993).Research findings suggest that teachers andtextbook authors often use analogies, but use

them ineffectively (Duit, 1991; Duit & Glynn.1992; Glynn, Britton, Semrud-Clikeinan, &Muth, 1989; Halpern, 1987; Halpern, Hansen.& Riefer, 1990; Thagard, 1992; Thiele &Treagust, 1991; Vosniadou & Schommer,1988). Sometimes the analogies that teachersand textbook authors use to explain new con-cepts are clear and well developed, while atother times they are vague and confusing.Teachers and textbook authors need a model toguide their construction of instructional analo-gies systematically. Analogies could then hecustom-tailored to suit students' own back-ground knowledge. In response to this need.research on a model for teaching with analo-gies was initiated (Glynn, 1991, 1989a.b:Glynn et al., 1989; Harrison & Treagust.1993; Thiele & Treagust, 1991).

Definition of Analogy

An analogy is drawn by identifying similaritiesbetween two concepts. In this way, ideas canbe transferred from a familiar concept to anunfamiliar one. The familiar concept is calledthe analog and the unfamiliar one the target.Both the analog and the target have features(also called attributes). If the analog and thetarget share common or similar features, ananalogy can be drawn between them. An ab-stract representation of an analogy, with itsconstituent parts, appears in Figure I . As canbe seen in Figure 1, the analog and target areoften examples of a superordinate (higher-order or cross-domain) concept.

An example of an analogy can he found inthe high school textbook Physical Science(1988) in which the familiar concept bookcase


1c


ANALOG

Features 1,2,3, etc.

SUPERORDINATE CONCEPT

compared with

*--->

TARGET

Features 1,2,3,

BOOKCASE(Analog)

Books

(Feature)

Number of Books per Shelf

Shelves

Number of Shelves

Distances Between Shelves

Floor

TIERED SHELVING SYSTEM(Superordinate Concept)

compared with BOHR'S ATOM(Target)

Electrons(Feature)

Number of Electrons per Level

Energy Levels

Number of Levels

Distances Between Levels

Nucleus

Figure I. Upper box. An abstract representation of an analogy showing the relation of the superordinate

concept to the analog and target and relation of the analog and target to the features. Lower box. An analogy

between a bookcase and Bohr's atom that follows the abstract representation shown in the upper box.

NATIONAL READIN3 RESE ARCH CENTER, READING RESEARCH REPORT NO. 15

6 Shawn M. Glynn

Energy Levels

Figure 2. In Bohr's model of the atom, electrons are located in energy levels like shelves in a bookcase.

is the analog and the concept Bohr's model ofthe atom is the target. In the context of Ru-therford's earlier model and Schrodinger's laterone, Bohr's model is explaineo in the followingway:

According to the Bohr model, the electronsin an atom are not whirling around thenucleus in a random way. Instead, electronsmove in paths. Each path is a certain dis-tance from the nucleus. Compare Bohr'smodel to a bookcase.... Just as each shelf isa certain distance from the floor, each pathis a certain distance from the nucleus. Also,the distance between one path and the nextis not the same for each path, just as theshelves in some bookcases are not the samedistance from each other. The paths in

electrons circle the nucleus arecalled energy levels. Electrons are found inenergy levels, not between them, just asbooks are found on bookcase shelves, not

floating between shelves. (Alexander et al. ,1988, p. 85)

An analogy can be drawn between the book-case and the atom because they share thesimilar features mapped in Figure 1. While aword mapping of features is sometimes suffi-cient for an analogy to be drawn, additionalgraphic or pictorial mappings are desirablebecause they activate the cognitive process offorming mental images, which helps studentsform better representations of the analogy.Figure 2 depicts the analogy in visual form.

The analog (bookcase) and the target(Bohr's atom) are examples of a superordinateconcept we call a tiered holding system. Often,it is hard to identify and label the superordinateconcept in an analogy that spans differentconceptual domains. In such cases, teachersand textbook authors should try to create averbal label, even when it sounds a bit pecu-liar.


4


Figure 3. A stair-step representation of possibleelectron transitions.

It is worth the trouble to identify and namethe cross-domain superordinate concept be-cause that concept can suggest subordinateconcepts that can also be used as analogs. Forexample. concepts subordinate to tiered hold-ing system include ladder and staircase. De-pending upon the purposes of the teacher orauthor, these could be better analogs than abookcase.

No analog matches the target perfectly.Every analog breaks down at some point andno two analogs are alike. Each analog has itscorresponding and noncorresponding features,and some will be better for some purposes thanothers. For these reasons, teachers and authorsshould try to suggest several analogs to stu-dents. For example, if the purpose is to dem-

onstrate, by analogy, how an atom emits andabsorbs light, a staircase analog can be moreuseful than a bookcase.

In their article, "The Stair-Step Atom,"Jordan, Watson. and Scott (1992) describe how

they use a specially constructed set of stairsteps when teaching university general-studiesastronomy and physics classes. The set of stair

steps is an analog for a generic atom; theinstructor taking the role of an electronjumps from a lower step to a higher one or viceversa (see Figure 3). The authors explain:

Each step represents a possible energy levelthat an electron can have in our atom. Thelowest energy level is on the floor andrepresents the ground state. The highestenergy level is located at the top step. Theregion between the steps represents thedifferences in the energies of the two lev-els. This region is painted with a color thatrepresents the wavelength of the energy

the photon emitted by the atom as anelectron changes orbits or steps. On thefront is the color of the transition to thenext step (state). On the side are colors thatrepresent wave-lengths of photons emitted

by the atom as electrons change their orbits(step) to the ground state, as well as toother energy levels. (p. 42)

Our stair-steps model, like an atom,has discrete energy values.... The personplaying the role of the electron must be one

step (energy level) or another, not between

steps. Our generic atom, in other words, isquantized. This simple analogy gives stu-dents a better understanding of what isgoing on inside an atom. (p. 44)

An instructor plays the role of anelectron in an atom that is undergoing the

process of de-excitation. When he jumpsfrom the top step to the middle step, he will

emit a photon (throw a ping-pong ball) with


8 Shawn M. Glynn

the energy and wave-length (color) equiva-lent to the difference between the twosteps. (p. 44)

Jordan et al. conclude by cautioning readersthat their stair-step analog is simple and farfrom a true representation of an atom. At thesame time, they believe the analog serves itspurpose well in demonstrating how energy isreleased during de-excitation in an atom.

In summary, when students are providedseveral analogs such as the bookcase andstaircase analogs of the Bohr atom, they canfocus on the target concept from several per-spectives and thereby come to a more completeunderstanding of it. Furthermore, when stu-dents are thinking in terms of several analogs,they are less likely to equate any one analogwith the target. The additional analogs need notbe developed in the same detail as the primaryanalog. A sentence or two emphasizing theirunique features will often be sufficient.

The identification and labeling of a super-ordinate concept can prompt students to gener-alize and transfer what they have learned toother related concepts, not just features. Thatallows us to expand the earlier definition of ananalogy: an analogy is a process of identifyingsimilarities among two or more concepts. Inother words, an analogy may involve an entirefamily of concepts, with concepts varying inthe degree of family resemblance they shareamong themselves.

Analogical Transfer: Bridging Concepts

An effective analogy puts new ideas into termsalready familiar to students. An analogy drawnby a teacher or author between a concept

covered earlier in a course and one coveredlater is particularly effective because there issome assurance that the earlier concept isfamiliar to every student. For example, inConceptual Physics, Hewitt (1987) explains theconcept gravitational potential energy in Chap-ter 8 and uses it again in Chapter 33 to intro-duce students to the concept electrical potentialenergy:

Recall from Chapter 8 the relation betweenwork and potential energy. Work is donewhen a force moves something in the direc-tion of the force. An object has potentialenergy by virtue of its location, say in aforce field. For example, if you lift anobject, you apply a force equal to itsweight. When you raise it through somedistance, you are doing work on the object.You are also increasing its gravitationalpotential energy. The greater the distance itis raised, the greater is the increase in itsgravitational potential energy. Doing workincreases its gravitational potential energy.

In a similar way, a charged object canhave potential energy by virtue of its loca-tion in an electric field. Just as work isrequired to lift an object against the gravita-tional field of the earth, work is required topush a charged particle against the electricfield of a charged body. (It may be moredifficult to visualize, but the physics ofboth the gravitational case and the electricalcase is the same.) The electric potentialenergy of a charged particle is increasedwhen work is done to push it against theelectric field of something else that ischarged. (pp. 501-5r2)



ally, an analogy effectively drawn betweentwo concepts will help students transfer theirexisting knowledge to the understanding, or-ganizing, and visualizing of new knowledge.The result is often a higher-order, relationalunderstanding; that is, the students see how thefeatures of a concept fit together and how theconcept in question connects to other concepts.Thus, students will be more likely to generalizetheir understanding to a superordinate concept,such as that of potential energy. They also will

be more likely to transfer their understandingto other instances of the superordinate concept,such as elastic potential energy and chemicalpotential energy, and see the similarities amongdifferent examples of potential energy, such asa lifted stone, a charged battery, a drawnarrow, and an unstruck match.

Effectiveness of an Analogy:Shared Features

The effectiveness of an analogy generallyincreases as the number of similar featuresshared by the analog and target increases. Forexample, teachers and textbook authors havetraditionally used the camera as an analogwhen teaching about the human eye, becausethe two concepts have so many similar fea-

tures. Consider the following excerpts from

Hewitt's (1987) Conceptual Physics:

In many respects the human eye is similar

to the camera. The amount of light thatenters is regulated by the iris, the colored

part of the eye which surrounds the open-ing called the pupil. Light enters throughthe transparent covering called the cornea,

passes through the pupil and lens, and is fo-

cused on a layer of tissue at the back ofthe eye the retina that is more sensi-tive to light than any artificial detectormade.... In both the camera and the eye,the image is upside down, and this is com-pensated for in both cases. You simply turnthe camera film around to look at it. Yourbrain has learned to turn around images it

receives from your retina!A principal difference between a cam-

era and the human eye has to do with fo-cusing. In a camera, focusing is accom-plished by altering the distance between the

lens and the film. In the human eye, mostof the focusing is done by the cornea, thetransparent membrane at the outside of the

eye. Adjustments in focusing of the image

on the retina are made by changing thethickness and shape of the lens to regulateits focal length. This is called accommoda-tion and is brought about by the action of

the ciliary muscle, which surrounds the

lens. (pp. 450-451)

The analogy between the camera and the eye is

a powerful one because the analog and thetarget share many features. The camera is aneffective analog, however, only when students

are familiar with its features. Hewitt (1987)ensured his students' familiarity with the fea-

tures of the camera by explaining its compo-

nents in a section titled "Some Common Opti-

cal Instruments" that preceded the section on

the eye.Another powerful analogy, one with many

corresponding features, is that between amechanical pump and the human heart. Many


10 Shawn M. Glynn

authors and teachers assume that the studentsare familiar with the features of a pump. Thisassumption is often unjustified. Students mightwell be familiar with the pump-to-heart analo-gy, but that doesn't mean they have a clearunderstanding of how a pump works. Regard-less of an analog's many shared features andtime-honored connection with a target concept,authors and teachers should always take thetime to ensure that students are familiar withthe features of the analog before they attemptto draw an analogy.

It is possible to draw a good analogy on thebasis of a few similar features, or even onefeature, if it is directly relevant to the goals ofthe teacher or author. For example, considerthe following analogy from Smallwood andGreen's (1968) chapter on "The Molecules ofLife" in their high school textbook, Biology:

Most carbohydrate molecules can be com-pared to a freight train that is made up ofboxcars linked together. Carbohydratemolecules are usually long chains of simplesugars bonded together. We call these largecarbohydrate molecules polysaccharides.(p. 54)

The preceding analogy is simple; the train andboxcars correspond to the carbohydrate mole-cule and the sugars, respectively. Despite itssimplicity, the analogy is effective because itmaps a familiar mental picture (a freight train)onto the target concept (a carbohydrate mole-cule). The analogy helps the students to visual-ize quickly the general structure of the mole-cule.

The instructional value of an analogy de-creases if it is difficult to identify and map theimportant features shared by the analog and thetarget. For example, consider this explanation

of gravity and "black holes" in a physicalscience textbook:

To understand black holes, astronomersstudy what happens when gravity is verystrong. An object with a huge mass hassuch strong gravity that it bends spacearound it. The curved space bends any lightthat passes by. (Pasachoff, Pasachoff, &Cooney, 1983, p. 466)

The teacher's edition to this textbook providesteachers with the following question and analo-gy, presumably to aid them in their explanationof the effects of gravity:

Is space curved or does gravity "pull on"the light? Analogy: two men walk side byside, ten feet apart, each perpendicular tothe equator. To their surprise, they collideupon reaching the north pole. Question:Did gravity pull them together, or is theearth curved? (Pasachoff et al., 1983, p.

467)

The answer to the preceding question is clear:the earth is curved. But what is not clear ishow the features of the analog two menwalking parallel to each other and perpendicu-lar to the equator until they collide at the northpole correspond to the target concept, theeffect of gravity on space and light. In thisanalogy, there is a strong likelihood that mis-conceptions will be formed.

Misconceptions Caused by Analogy:The Dark Side

The advantage of teaching with analogy is thatit capitalizes on students' relevant existingknowledge. Learning becomes relational rather

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than rote, and therefore it is more meaningful.The process of joining new knowledge toexisting knowledge is intrinsically motivating.Analogical thinking is also efficient; it helps usto understand new phenomena and solve newproblems by drawing upon our past experienc-es. This is the bright side of analogical think-ing. There is a dark side as well. When stu-dents overgeneralize and map noncorrespond-ing features of concepts, misconceptions canresult (Thagard, 1992). This dark side ofanalogical thinking is an unfortunate fact oflife.

On the first day of a science course, somehigh school teachers and college professorsroutinely advise their students: "Forget whatyou think you know about how the worldworks. You'll learn how it really works in thiscourse." These professors and teachers have apoor understanding of how students learnmeaningfully. Meaningful learning requiresthat students' existing knowledge be taken intoaccount, not ignored. Conceptual bridges mustbe built between existing knowledge and newknowledge; analogy plays an important role inthe construction of these bridges. Trying toavoid analogy is, to use another analogy, likethrowing the baby out with the bath water. Weseem to have a natural bent for analogicalthinking. Even very young children reasonanalogically, as is illustrated by Jean Piaget's(1962) observation of his daughter Jacquelinewhen she was two years and 10 months old:

J. had a temperature and wanted oranges. itwas too early in the season for oranges tobe in the shops and we tried to explain toher that they were not yet ripe. They arestill green. We can't eat them. They have-n't yet got their lovely yellow color." J.seetr21 to accept this, but a moment later,

as she was drinking her camomile tea, shesaid, Camomile isn't green, it's yellowalready.... Give me some oranges. (p. 231)

Effective teachers and textbook authors capital-ize on analogical thinking rather than ignore it.They make systematic use of analogy andemphasize to students that analogical thinkingis powerful, but limited, and that wrong ideascan arise when an analogy is carried too far.For example, in their textbook Concepts inPhysics, Miller, Dillon, and Smith (1980)explain to students:

Models and analogies can be of great valuein physics if they are used with care anddiscrimination. It is important, for exam-ple, to guard against the danger of believ-ing that a model or analogy is an exactrepresentation of some physical system.One should always regard a model critical-ly and remember that an analogy means nomore than: under certain special conditions,the physical system being studied behavesas if.... (p. 253)

Analogies are double-edged swords. Ananalog can be used to explain and even predictsome aspects of the target concept; however, atsome point, every analogy breaks down and, atthat point, misconceptions may begin. Sincetwo concepts are never completely identical,differences always exist among their definingfeatures. For example, consider an analogyused to explain drug overdose from a chapteron "Drugs and Behavior" in the high schooltextbook Biology: An Everyday Experience(Kaskel, Hummer, & Daniel, 1988). One ofthe accompanying figures is of a partially stop-pered sink, with water flowing into the sinkand out of it at the same rate. Another figure


1St

12 Shawn M. Glynn

shows an overflowing sink, with more waterflowing into it than can drain out. The expla-nation in the text reads:

The body balances the amount of drugentering and leaving it. Think of the watergoing into the sink as a drug ent :ring thebody. The leaking stopper stands for theorgan removing the drug. If the waterentering the sink is equal to the waterleaving it, the water will not overflow. Norwill the sink become empty. The same typeof thing happens with a correct drug dose.With the proper drug dose, the amount ofdrug entering the body equals the amountleaving it.

What happens if a person does not takethe correct drug dose? A drug overdosecould result. An overdose is the result oftoo much of a drug in the body. Let's lookat the sink example again. [The figure]shows what might lead to a drug overdose.Too much of a drug is added to the body.The body cannot get rid of the drug fastenough, and a drug overdose results. (p.332)

The preceding analogy, drawn between waterin a sink and a drug in the body, could easilylead to a misconception. While the authorsnote, "Drugs taken into the body soon leave itor change into a different form," the analognevertheless promotes the misconception that aprescription or nonprescription drug flowsthrough the body without interacting physicallyand chemically with its constituents. Unlesscautioned otherwise by the authors or teachers,students could assume that a drug such as

alcohol poses few risks if it flows through themand soon exits in urine.

A careful examination of all features of ananalogy is a prerequisite to using it effectivelyin instruction. When teachers and authors usean analogy, they should anticipate analogy-caused misconceptions and eliminate them bypointing out to students where the analogybreaks down.

TEACHING -WITH- ANALOGIES MODEL:RESEARCH AND DEVELOPMENT

Research on a model for teaching with analo-gies began with a task analysis of middleschool, high school, and college science text-books; the analysis identified how the authorsof 43 textbooks used analogies to explain newconcepts to students. The task analysis of text-books has now been supplemented by an analy-sis of the lessons of exemplary science teach-ers.

Task Analysis: A Research Method

Task analysis was used in the present researchwith the intent of modelling an individual

expert's thinking" (Wiggs & Perez, 1988, p.267). Task analysis (Gagne, 1985; Gardner,1985), also called a procedural analysis, is atechnique that "identifies and structures thebasic processes that underlie task perform-ance.... You are trying to document the basicprocesses that are involved in performing acognitive task" (Goetz, Alexander, & Ash,1992, p. 360). A task analysis of how expertsperform a cognitive task leads to a representa-tion of the experts' knowledge and, eventually,



Table 1. Operations in the Teaching-with-Analo-gies Model:

I. Introduce target concept.2. Cue retrieval of analog concept.3. Identify relevant features of target and analog.4. Map similarities between target and analog.5. Indicate where analogy breaks down.6. Draw conclusions.

to a model of the task that includes the opera-tions carried out in the performance of thetask. For example, a task analysis of expertwriters led Flower and Hayes (1981) to amodel of competent expository writing; themodel has subsequently played an importantrole in the instruction of novice writers (Hayes& Flower, 1986). Likewise, in the presentresearch, task analyses of science textbooksand exemplary teachers were expected to leadto a model for teaching with analogies a

model that could provide less experiencedauthors and teachers with guidelines on how touse analogies effectively.

Research Findings: Textbooks

A task analysis was conducted on the analogiesin 43 middle school, high school, and collegetextbooks (see Glynn, 1991; Glynn et al.,1989). The sentences comprising the analogiesin each textbook were examined. For eachsentence or group of related sentences (calledstatements) the question was asked: "What isthe author's intention here?" This question.alternatively stated, is: "What operation is theauthor performing'?" The statements were

sorted into categories. A summary of thesedata for all textbooks indicated that the state-ments fell into six main categories, each corre-sponding to an operation. The operations thatunderlie these categories are shown in Table 1.Together, these six operations form the basis ofthe Teaching-with-Analogies Model. The orderof the six operations listed in Table 1 is onlyapproximate. The authors actually varied withrespect to the order in which they carried outoperations, the number of operations theycarried out, and the number of times theycarried out any given operation.

While all of the textbook authors wereconsidered to be experts, the analysis identifiedthe analogies drawn by Hewitt (1987), authorof Conceptual Physics, as being among thebest. He consistently uses the six operationslisted in Table 1, and draws analogies, not onlybetween concepts, but between principles andformulas as well. For example, in ConceptualPhysics, he draws a detailed analogy betweenNewton's law of gravitation and Coulomb'slaw of electrical force. A graphic map of thisanalogy appears in Figure 4.

In his text, Hewitt introduces the targetconcept, Coulomb's law, by reminding stu-dents of the analog concept, Newton's law,which he had explained in an earlier chapter:

Recall from Newton's law of gravitationthat the gravitational force between twoobjects of mass mi and mass m2 is propor-tional to the product of the masses andinversely proportional to the square of thedistance d between them: F = G(nlm2Id2),where G is the universal gravi ,tionalconstant. (pp. 482-483)


14 Shawn M. Glynn

attracts

product of masses

I

small magnitudeconstant

attracts or repulses

product of charges

iq2qF-k

large magnitudeconstant

inverse-squares

d2

Figure 4. A comparison of two inverse-square laws: Newton's law of gravitation and Coulomb's law ofelectrical force.

Hewitt does not just remind students of theanalog, Newton's law; he briefly re-teaches it,identifying the key features and relations in theanalog. Hewitt knows that some students willforget Newton's law, entirely or in part, andthey will not take the time to flip back to theearlier chapter. So, he refreshes their memoryfor them. Next, Hewitt identifies the featuresand relations of the target concept, Coulomb'slaw:

The electrical force between any two ob-jects obeys a similar inverse-square rela-tionship with distance.... Coulomb's lawstates that for charged particles or objectsthat are small compared to the distance be-tween them, the force between the chargesvaries directly as the product of the chargesand inversely as the square of the distancebetween them. The role that charge plays in

electrical phenomena is much like thethat mass plays in gravitational phenomena.Coulomb's law can be expressed as: F =k(q,(721d2) where d is the distance betweenthe charged particles: q, represents thequantity of charge of one particle and q, thequantity of charge of the other particle; andk is the proportionality constant. (p. 483)

Then, Hewitt maps the similar features andrelations of the concepts. He is careful to pointout where the analogy breaks down. By doingso, he reduces the likelihood that students willovergeneralize from the analog to the targetconcept and form misconceptions:

The proportionality constant k in Cou-lomb's law is similar to G in Newton's lawof gravitation. Instead of being a very smallnumber like G, the electrical proportion-

NATIONAI. READING RESEARCH CENTER, READING RESEARCH REPORT NO. 15

2i


ality constant k is a very large number....So Newton's law of gravitation for massesis similar to Coulomb's law for electriccharges. Whereas the gravitational force ofattraction between a pair of one-kilogrammasses is extremely small, the electricalforce between a pair of one-coulomb charg-es is extremely large. The greatest differ-ence between gravitation and electricalforces is that while gravity only attracts,electrical forces may either attract or repel.(pp. 483-484)

Finally, Hewitt draws main conclusions forstudents about both the target concept (electri-cal force) and the analog (gravitational force):

Because most objects have equal numbersof electrons and protons, electrical forcesusually balance out. Any electrical forcesbetween the earth and the moon, for exam-ple, are balanced. In this way the muchweaker gravitational force, which attractsonly, is the predominant force betweenastronomical bodies.

Although electrical forces balance outfor astronomical and everyday objects, atthe atomic level this is not always true. Thenegative electrons of one atom may at timesbe closer to the positive protons of a neigh-boring atom than to the electrons of theneighbor. Then the attractive force betweenthese charges is greater than the repulsiveforce. When the net attraction is sufficient-ly strong, :.toms combine to form mole-cules. The chemical bonding forces thathold atoms together to form molecules areelectrical forces acting in small regions

where the balances of attractive and repel-ling forces are not perfect. (pp. 484-485)

To illustrate the implications of his conclu-sions, Hewitt provides students with a concreteexample in which he compares the electricaland gravitational forces between the proton andthe electron in a hydrogen atom. By means ofthis example, he shows students that the electri-cal force in the hydrogen atom is much greaterthan the gravitational force. So much so, thatthe gravitational force is negligible. Hewitt alsopoints out to his students that the similaritybetween the law of gravitational force and thelaw of electrical force may point the way tonew discoveries in science:

The similarities between these two forceshave made some physicists think they maybe different aspects of the same thing.Albert Einstein was one of these people; hespent the later part of his life searchingwith little success for a "unified field theo-ry." In recent years, the electrical forcehas been unified with the "weak force,"which plays a role in radioactive decay.Physicists are still looking for a way tounify electrical and gravitational forces. (p.484)

Hewitt's use of analogies is exemplary, butmany authors use analogies ineffectively. Thetask analysis of 43 science textbooks revealedsome instances in which authors suggested asketchy analogy to students, and then aban-doned the analogy, leaving students to makesense (or nonsense) out of it for themselves.Under these conditions, students could identifyirrelevant features of the target and analog


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16 Shawn M. Glynn

concepts, map these features, fail to realizewhere the analogy breaks down, and drawwrong conclusions about the target and analog.In other words, the misguided use of analogiesby some authors could actually promote mis-conceptions in students.

Research Findings: Teachers

The task analysis of textbook analogies wassupplemented by a task analysis of the lessonsof 10 exemplary science teachers (see Glynn,1993; Glynn, Law, & Gibson, 1994). Theexemplary science teachers were from publicmiddle schools. The teachers were identified asexemplary by means of awards and the judg-ments of principals, other teachers, and univer-sity teacher educators. All classes were multi-cultural, with 18 to 25 students in each class.The lessons were observed and videotaped; atask analysis of the lessons was conducted. Theteachers later re-enacted their lessons in avideo studio and were asked to explain thedecisions they made during the course of theirlessons.

Each teacher selected a lesson in which heor she made the best possible use of analogy-based activities to elaborate on a key conceptthat the students had read about in their text-books. For example, one of the teachers,Martha Gilree, taught an earth science lessonon the structure of the earth. She baked layeredcupcakes for her students and explained thatthe cupcakes were analogs of the earth, withthe four layers corresponding to the crust,mantle, outer core, and inner core of the earth.Using straws, the students took "core samples"from the cupcakes, examined the samples, and

compared them to representations of the earthin their textbooks.

Another teacher, Joe Conti, taught a biolo-gy lesson on natural selection and the conceptof survival of the fittest. He took his studentsoutside to an area of green grass where earlierhe had scattered an equal number of green,brown, and red uncooked noodles. He exp-lained that the noodles were analogs for differ-ent colored grasshoppers and that the studentswere hungry birds who preyed on the grass-hoppers. The students "caught as many grass-hoppers" as they could in the next five minutesand returned to the classroom where a tallyrevealed that fewer green grasshoppers werecaught than brown, and fewer brown than red.Joe then explained to his students how a traitsuch as coloration can increase or decrease theprobability a species will survive in a givenenvironment.

Still another teacher, Becky Wheeler,taught a physical science lesson on optics. Sheand her students built a simple, working cam-era and she used this camera to explain opticprinciples (see Table 2). Becky then explainedthat the camera is an analog for the human eyeand, using a physical model of the eye, shecompared its features to those of the camera.Finally, she taught optic principles common tothe camera and eye.

A task analysis was carried out on thevideotaped statements and actions that made upthe lessons of the 10 exemplary teachers. Ineach lesson, a statement was one sentence or afew related sentences on either a target concept(e.g., the eye), an analog concept (e.g., thecamera), or a conceptual feature (e.g., thelens). For each statement, the question was



Table 2. Becky Wheeler's Lesson on Optics

Statements Operations

One of the things we're going to learn today is what the eye looks like. 1

If I were to take out my eye, anyone else's eye and put it on a plate,they'd look pretty much the same; they'd be about the same size.[Teacher shows ping-pong ball.]

2/3

What do you call the eye, generally? How would you refer to the eye if 2you took it out like this? Yes, an eyeball.

We call it a ball because it's round and actually this ping-pong ball is 3/4about the same size as an eye.

How many of you think you know what this is? [Teacher shows a glass 2lens.]

Actually, if you wanted to know what the eye looks like, the lens of it 3

looks like this.

Now, it's not exactly the same material because the eye is made of soft 5

tissue and this is made of glass. It looks a lot like this but it's not thisbig; this is much bigger.

Okay, let's look at a model of our eye. 2[Teacher shows plastic model of eye.]

We can see this white part, which is sort of a tough tissue and it's called 3

the sclera.

You can see this bulging part...put your finger on your eye and move it 3

right and to the left. Can you feel the bulge? Who knows what that bulgeis called? Yes, the cornea.

The part that goes around the pupil and changes the size of the hole isthe iristhat controls the amount of light and changes the size of thepupil.

3

Actually, it can get as tiny as the end of a pencil and as large as aboutthe size of the eraser on the other end. [Teacher shows and comparespencil features.]

2/4

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18 Shawn M. Glynn

The clear transparent part on the front is called the aqueous humor. 3

Inside your eye is a gel-like solution called the vitreous humor and ithelps to give the eye its shape.

The light has to come in; it goes through the pupil...and finally it gets to 3

the back of the eye. What is that called? Okay, the retina.

A lot of times we compare the eye to a camera.

Think about the parts we just talked about...an eyelid...what could we 4/6

compare the eyelid to on this camera? Yes, the shutter.[Teacher shows and compares camera.]

On the camera, what do you have that controls the light that goes into it, 4/6

comparable to the iris that controls the amount of light going into theeye? Yes, the aperture.

The lens, we said, is an important part of the eye: and of course, the 4

camera has a lens.

You told me that you have a retina on the back of your eye that's light-sensitive. What do you have on the back of the camera that's light-sensitive? Okay, light-sensitive paper the film.

4/6

On our retinas we have special nerve cells that can pick up black andwhite color. These are called rods, and we have sonic cells called conesthat pick up colors. So on a camera, what do you think we could use tocompare to the rods?

4/6

Now the optic nerve in our eye takes the message to the brain; you knowthe camera doesn't have that.

5

Okay, the vitreous humor that's part of the eye, but it's not a part ofthe camera.

5

Does anyone know what I have here? A pinhole camera. [Teacher showshomemade, tube-like pinhole camera.]

1

What do you think this part, the shutter, would he compared to? Yes,the eyelid.

4/6

Then we've got the little pinhole, the aperture; what could we compare 4/6

that to? The pupil, the iris working together.


26


On the very back, we have the light-sensitive paper; what part of the eye 4/6would you compare that to? Yes, the retina.

Okay, there's a wonderful part of your eye that controls the lens so 5

that the lens in your eye is different from the lens in a camera. In thelens in a camera you have to have a way of adjusting it to make it focus,but the lens in our eye has special muscles called ciliary muscles. Theycan make the lens flat and they can make the lens thin.

Okay, the retina on the back of the eye is different from the film of the 5

camera because it's very small...

...it's actually about the size of a postage stamp. 2/4

All of the nerve fibers come together and form something like a cable; 2/4that is where the optic nerve attaches.

[Teacher shows poster of traffic signal and eye; string represents lightrays.] All of the light rays are coming from this red light...when itcomes it's straight and when it gets to this area, it is going to bend sothat it ends up here on the back of the eye.

2

Now, what has happened to the image? Yes, it's turned upside-down. 6

[Teacher shows a glass lens.] This is just like the lens in your eye: lighttravels in a straight line, comes through the lens...

2

Look at me through this lens; how do I appear to you? [Students look 6and see upside-down image of teacher.]

Note. See Table 1 for Operations 1 through 6.

asked: "What operation is the teacher per-forming?" The statements were sorted intocategories, tallied, and summarized for allteachers. The teachers were found to use thesix operations that the textbook authors did, aswell as several others. For example, someidentified concepts that are superordinate to thetarget and analog concepts, such as: The sci-ence of optics is superordinate to the conceptseye and camera. These other operations were

relevant to the analogies, but were unessentialand performed infrequently. None accountedfor more than 2% of the statements. Becausethey were unessential and infrequent, theseother operations were not added to those al-ready in the Teaching-with-Analogies Model(see Table 1).

Thirty-five sample statements and thecorresponding operations for the lesson onoptics taught by Becky Wheeler are presented


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20 Shawn M. Glynn

in Table 2. (The operation numbers in Table 2correspond to those in Table 1.) Becky Wheel-er, like the other teachers, carried out theoperations in an order that roughly correspondsto the order in Table 1. Two independent ratersviewing the videotapes agreed 86% of the timeon the categorization of the transcribed state-ments; the raters resolved the other 14%through discussion. The teachers' statements,which were spontaneous and oral, were moredifficult to categorize than the written state-ments of the authors. The content and grammarof the teachers' statements were less structuredand precise than those of the authors. In addi-tion, the teachers' statements frequently includ-ed questions that the students were asked toanswer; after the students answered, the teach-er then provided feedback. For example,Becky Wheeler mapped similarities (operation4) between an eye and a camera and askedstudents to draw conclusions (operation 6):"Think about the parts we just talked about...an eyelid...what could we compare the eyelidto on this camera? [The students responded"Shutter!" ] Yes, the shutter." As a result ofthis interactive questioning, two-operationstatements were more common among teachersthan authors.

As can be seen in Table 2, the primaryanalogy in Becky Wheeler's lesson on optics isdrawn between a camera and the human eye;however, it is important to note that there aresecondary analogies as well. These includecomparing the eyeball to a ping-pong ball, theoptic nerve fibers to a cable, and the eye itselfto a plastic model and to a traffic light. Theother teachers' lessons were similar in thisrespect: each included a primary analogy, but

also made use of several related, secondaryanalogies.

DISCUSSION

The task analyses of textbook authors andexemplary teachers indicated that, in practice,the order in which the six Teaching-with-Anal-ogies operations are carried out can vary. It isusually important, however, for the teacher ortextbook author to perform all of the opera-tions. If the teacher or textbook author were toperform only some of the operations, leavingsome to the student, it is possible that thestudent might fail to perform an operation ormight perform it poorly. The result could hethat the student will misunderstand the conceptbeing taught.

It is reasonable to assume that teachers andtextbook authors will use the process of analo-gy more effectively if they keep the Teaching-with-Analogies Model in mind. They canmentally check off the six operations in themodel when constructing an analogy to explaina new concept. When an author doesn't per-form some of the operations in the model andthe analogy suffers as a result, the teacher canperform them for the students. Or, when theauthor's analogy is effective and the teacherwants to extend it, the teacher can use themodel for this purpose, thus increasing theinstructional value of the analogy.

Recently, Harrison and Treagust (1993)carried out an extensive case study in which anexperienced science teacher was trained withthe Teaching-with-Analogies (TWA) Modeland used it to teach a lesson on optics and the


2G


refraction of light. For purposes of teachertraining, Harrison and Treagust modified themodel by reversing the order of the final twooperations (see Table 1). As noted previously,the order of the operations does vary in prac-tice: however, it may be that one order isbetter than others for purposes of introducingthe model to teachers. Regarding the effective-ness of the model, they concluded:

Based on the data collected over four les-sons ... it is asserted that in this instance,with this teacher, a systematic approachlike the modified TWA model is a practicaland achievable means for improving the useof analogies in science classrooms.... Atthe conclusion of three episodes duringwhich Mrs. Kay used the modified TWAmodel, she appeared to be competent andfluent in its use and the students respondedpositively to the resultant analogical in-struction. As shown by this data, there wasa consistently high level of understandingencountered during the interviews. In thedescribed lesson, Mrs. Kay felt that thestudents had understood refraction betterthan on any previous occasion that she hadtaught this concept. Based on our studieswith this teacher and five other teachers, itis predicted that the majority of practicingteachers would require extended practicecombined with critical feedback in order tointegrate the modified TWA model intotheir pedagogical repertoire. (pp. 1293)

Teachers also are encouraged to use theTeaching-with-Analogies Model to constructadditional analogies to complement an author's

analogy. Several analogies constructed for thesame concept can help the students view thetarget concept from different perspectives. Theanalogies function like conceptual lenses, witheach one bringing different features of theconcept into sharper focus. For example,several analogies can be used to discuss viewsof how galaxies are arranged in the universe:

Astrophysicists closing in on the grandstructure of matter and emptiness in theuniverse are ruling out the meatball theory,challenging the soap bubble theory andputting forward what may be the strongesttheory of all: that the cosmos is organizedlike a sponge.

This new concept holds that a surpris-ingly complex arrangement of clusteredgalaxies stretches in connected tubes andfilaments from one end of the universe tothe other and that galaxy-free voids form anequally complex, equally well-connectedstructure.

Far more rapidly than was possible afew years ago, scientists are assemblingdata from the most distant galaxies to pro-duce a picture of the universe's structure.The sponge idea is meant to resolve a clashbetween views of the universe as clumps ofmatter on an empty background (the meat-ball concept), or as empty voids carved outof a full background (the bubbles concept).("Sponge Concept," 1986, p.7).

All three analogies meatball, soap bubble,and sponge are useful in explaining views ofhow galaxies are arranged. These views aresimilar but have important differences. Each


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22 Shawn M. Glynn

analogy, with its particular constellation of fea-tures, has its own explanatory power.

FUTURE DIRECTIONS FOR RESEARCH:STUDENT-GENERATED ANALOGIES

The research described in this report has fo-cused on how teachers and textbook authorscan construct effective analogies to help stu-dents comprehend textbook concepts. Futureresearch will also focus on determining howstudents can construct effective analogies forthemselves, independently of teachers andtextbook authors. Research and developmentwill begin on a variation of the Teaching-with-Analogies Model, called the Learning-with-Analogies Model. It is anticipated that studentswho are taught how to use a Learning-with-Analogies Model will be able to interpret.criticize, and extend analogies provided byauthors and teachers.

A lively class discussion in which an analo-gy is dissected by students who understand theLearning-with-Analogies Model could helpstudents to better understand the target conceptand, at the same time, help the teacher todiagnose misconceptions the students mighthave. For example, middle-school teacherssometimes compare the process of photosynthe-sis to baking a cake (Glynn, 1989b). Thisanalogy helps teachers identify those studentswho mistakenly believe that plants get theirfood directly from the soil (i.e., use plant food)rather than make their own food from rawmaterials in the air and soil (i.e., water andcarbon dioxide). Many students have seensomeone baking a cake and are thereforefamiliar with the analog. By asking students to

explain how baking a cake is like photosynthe-sis, teachers can identify students who havemisconceptions about photosynthesis. Teacherscan then use the analog to correct the miscon-ception. This analogy, like all analogies,breaks down at various points, and teacherscan use these points to further assess students'true understanding of the target, photosynthe-sis.

Students could also use a Learning-with-Analogies Model as a guide when constructingtheir own analogies. For example, studentsseeking an alternative to the baking-a-cakeanalog for photosynthesis might use a building-a-house analog to construct their own analogy:Raw materials (lumber + nails + shingles) +energy from a carpenter --0 final product(house) + waste products (sawdust + scrap-wood). The final and waste products corre-spond to sugar and oxygen, respectively (seeKaskel, Hummer, & Daniel, 1988, pp. 360362, for a detailed version of this analogy).

Even when authors and teachers haveprovided analogies for a particular concept, itis advantageous for students to construct theirown analogies because students must then usetheir own relevant knowledge. This ensuresthat the analogies will be meaningful. In addi-tion, students who can construct their ownanalogies become more independent in theirlearning. They can tackle new concepts ontheir own, using analogical reasoning as a wayof understanding those concepts.

Author's Note. For information on how to obtainthe NRRC Video Highlight, "Teaching Science withAnalogy: Building on the Book," please write to the


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National Reading Research Center, 318 AderholdHall, University of Georgia, Athens, GA 30602.

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