In four experiments with three-year-olds (N= 67), we investigate children's understanding of the differential importance of shape for categorization of solid rigid objects with fixed shapes and solid but deformable objects with shapes that can be changed. In a non-naming task we find that young children categorize rigid and deformable things differently and know that material is important for deformable things and shape for rigid things. In two naming tasks, however, children generalize names for both solid and deformable objects by shape similarity and disregard rigidity. To understand this pattern of results we examine a corpus of early-learned nouns and the kinds of rigid and nonrigid things named by nouns in that corpus. The results suggest that names for categories of solid, rigid objects in which instances are similar in shape dominate children's early noun vocabularies. We suggest that children's novel word generalizations for deformable things may be overgeneralizations of this dominant pattern.