the most enduring and essential characteristics of materials (density, boiling and melting points, thermal and electrical conductivity, solubility, etc.) are not yet known to them (Johnson, 1996; Smith, Carey, and Wiser, 1985; Wilkening and Huber, 2002). Related to this, their knowledge construction and evaluation practices are based on casual everyday observations using their commonsense impressions, not on careful measurement, modeling, and extended argument. At the outset of school, they have had limited experiences using instruments to measure things, and they have even less understanding of the deeper reasons for using instruments or of explicit criteria for judging what makes a good measurement (Lehrer, Jenkins, and Osana, 1998).

Hence, the overall goal at the K-2 level is to have children clarify, extend, systematize, and even begin to problematize their understandings of common materials and important physical quantities (especially weight and measures of spatial extent). Curriculum developers need to be mindful that children are ready to tackle these issues, while at the same time realizing that they are still conceptually difficult for them. They need to realize that central to helping students make progress with these issues is not just providing them with new facts or experiences, but also introducing them to cultural tools and practices that enable them to extend and restructure their understandings.

One specific goal is to extend children’s knowledge of materials and help create a richer notion of material kind as a dense causal nexus: that is, to realize that objects are constituted of materials and have some properties because they are made of that material. For example, children can be presented with objects made of a range of different materials (or containers with a range of different liquids). They can be asked to organize, describe, and classify the things by the kind of material they think they are made of and to defend their classifications. They can be asked to describe the properties of the objects and compare them in their properties, using new tools for organizing their descriptions, such as Venn diagrams and attribute/value charts.

In addition, they can consider why objects behave as they do in situations that implicate the materials they are made from. For example, they can compare the properties of two cups (one made of plastic and the other glass) and consider how each will respond when dropped and why. Or they might compare the properties of two balls (one made of metal and another of rubber) and consider how they respond when dropped and why. They can be introduced to common names for certain materials and asked how they could tell if something else was made of that material. They can predict how the observable properties of things might change or stay the same if an object is reshaped, divided into little pieces, or heated until melted, and whether they think it will still be the same kind of material. They then carry out those transformations and record and interpret what happens. For ex-



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