Two studies explored how mothers and preschoolers talk and reason about events in which biological and nonbiological objects change in size. Analysis of conversations indicated that although mothers discussed events primarily in domain-specific ways when using the term growth, they exhibited some domain blurring in explanations to preschoolers. Analysis of preschoolers' open-ended discussions about objects changing in size suggested that preschoolers have begun to develop domain-specific understandings. (Author/KB)