ample, they might melt a chocolate bunny and be asked to describe how it has changed. They can also be asked whether they think it is still chocolate, whether it still has the same amount of stuff, whether it has the same weight, and to make arguments about how they can tell.

In the course of these activities, not only will they be learning about how to form meaningful classifications, to carry out simple investigations, and to represent and record data in useful ways, but also they will deepen their understanding of materials. They will learn that not all properties of chunks stay the same when cut into pieces and that there are at least two ways of trying to trace (or track) the identity of different materials over time—by historical continuity (i.e., by following where it came from, or what was done to it, such as grinding) or by consideration of its observable properties.2 Originally, children might be more inclined to use commonsense properties than historical tracing. Historical tracing is important (especially across decompositions, such as grinding into smaller pieces) because it helps build an explicit idea of a material as an underlying constituent. By engaging children with considerations of what happens to materials with decomposition, they come to identify materials not just by their common perceptual features, but as constituents that can maintain their identity (and certain properties) even when they become arbitrarily small. They also begin to realize that not all large-scale properties of materials are preserved during that decomposition (i.e., some emerge when there is enough stuff or under certain conditions).

Another (related) goal identified for this period is to extend children’s descriptions beyond commonsense perceptions—especially for important physical magnitudes like weight and volume—by engaging them with the problem of constructing measures for a variety of quantities so that they can develop an explicit theory of measure that underlies the practice of measurement. Measurement is an important scientific practice that contrasts with everyday practice and grows out of concern with having data that can be described in precise objectively reproducible (or verifiable) ways and made amenable to mathematical representation and manipulation. It also greatly aids in finding lawful patterns in data—patterns that would be totally obscured if one relied on commonsense impressions. Yet many aspects of the underlying logic of measurement are not initially obvious to students and can be hidden by simply teaching them how to use preexisting or standard measuring procedures or instruments. Thus, learning to measure should in-


Of course, as chemists can attest, the problem of tracking the identity of materials is complex, and neither of the strategies children use is infallible. In fact, in some situations (chemical reactions), one substance will cease to exist and another come into being. In addition, many of the observable properties that children use are not the most reliable or valid cues to material identity.

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