conscious theory-building on the child’s part, learning to measure volume and weight and, in some cases, developing an atomic-molecular framework of matter (Smith et al., 2006; Stavy, 1995).

Chemistry is a domain in which some of the clearest cases of dramatic conceptual change have been documented during childhood, at both the macroscopic and atomic-molecular levels. For example, although even preschool children understand that objects vary in their size and weight, their initial understanding of these magnitudes is grounded in their perceptual experiences, rather than an explicitly articulated theory of matter. Size is “perceived global bigness,” in which different spatial dimensions have not yet been differentiated. Weight is “felt weight,” in which weight and density have not yet been differentiated as distinct physical magnitudes. Children often judge that some light objects, like a small piece of Styrofoam, weigh “nothing at all” because they exert no force on their hand (Smith, Carey, and Wiser, 1985; Smith, Solomon, and Carey, 2005). Hence, their understanding of these properties undergoes dramatic change as they construct distinct measures of weight and volume, come to see them as central properties of all matter, and interrelate these quantities in a distinct concept of density tied to their notion of material kind (Lehrer et al., 2001; Smith, Carey, and Wiser, 1985; Smith et al., 1997; Smith, Solomon, and Carey, 2005).

Furthermore, when young children are beginning to develop an explicit concept of matter that includes both solids and liquids, it is initially grounded in more commonsense perceptual properties—something one can see, feel, or touch—rather than as something that takes up space and has mass (Carey, 1991; Stavy, 1991). Thus, children have difficulty recognizing that matter continues to exist when divided into pieces too small to see (Carey, 1991; Smith et al., 1997) and generally do not appreciate the material nature of gases (Lee et al., 1993; Stavy, 1988; Smith et al., 1997).

Even more challenging to their everyday experiences of matter and materials are the assumptions of the atomic-molecular theory of matter. Not only does this theory call for them to imagine matter at a scale far removed from their everyday experiences, but it also makes theoretical commitments that violate their metaphysical beliefs (e.g., there is no vacuum). Everyday experience—the experience that matter is continuous—is deeply entrenched, and the experience that the kinds of materials in the world are infinitely varied is not easily reconciled with the notion that there are only about 100 different kinds of atoms on earth (Nussbaum, 1985; Lee et al., 1993). However, although there is still much for children to learn, even in preschool they are making some distinctions between object and material levels of description in organizing their knowledge of the natural world. They are beginning to ask themselves what are things made of, what changes and what stays the same across different transformations, and learning to count things and even to build some initial causal accounts of why things have the

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