One important function of categories is to allow inferences that extend beyond surface appearances. In 2 studies, preschool and second-grade children were tested on their understanding that members of a category have similar internal parts. In 1 study, children were taught new information about the internal structure of various objects (e.g., an apple), then were probed to determine how far they generalized the new information (e.g., to another apple, to other fruit, to an unrelated object). In a second study, children participated in an open-ended interview that probed whether various types of objects had "the same kinds of stuff inside." Children at both ages and in both studies drew many inferences concerning the internal children to report that members of a basic-level category had the same internal parts. Older children drew more inferences at the superordinate level than did younger children. Older children were also more sensitive to differences in category domain (natural kind vs. artifact) at both basic and superordinate levels. Altogether, these results suggest that preschool children assume that basic-level categories share internal parts. They need to refine this belief at the basic level and to extend it to superordinate-level categories.