Young children distinguish between the physical and biological domains of knowledge. The current study examines how this distinction is expressed in a story construction task. Three- and 4-year-olds were shown pairs of pictures, one that depicted a normal event and one that depicted an event that violated either physical or biological causal structure. Children were asked to choose which picture to include in a story. Three-year-olds generally showed no systematic patterns of responses when constructing their stories. Four-year-olds, in contrast, made more normal than violation choices overall, and their stories were relatively consistent with regard to the fictional world's internal coherence. When 4-year-olds did include violation events in their stories, those violations tended to be of physical rather than biological causality. These data suggest that a general understanding of impossible events is underpinned by causal domain knowledge.