Two studies investigate children's knowledge of internal parts and their endorsement of immanent causes for the behaviors of living and non-living things. Study 1, involving 48 preschoolers, showed that domain-specific knowledge of internal parts develops between ages 3 and 4. Study 2 included 43 4-year-olds, 30 8-year-olds, and 35 adults and showed that preschoolers do not endorse these internal parts as causally responsible for familiar biological events (e.g., movement, growth). Like adults and older children, however, preschoolers endorse an abstract cause, “its own energy,” for animals but not for machines. The results suggest that children recognize domain-specific internal parts as early as age 4 but that their causal attributions are not yet anchored in a detailed biological theory. Findings are discussed in terms of theory change and an essentialist assumption.