Many researchers have long assumed imaginative play critical to the healthy cognitive, social, and emotional development of children, which has important implications for early-education policy and practice. But, the authors find, a careful review of the existing literature highlights a need for a better theory to clarify the nature of the relationship between pretend play and childhood development. In particular, they ask why children spend so much time engaging in unreal scenarios at a time when they know relatively little about the real world? The authors review the idea that children pretend because it exercises their developing ability to reason counterfactually--an ability essential for causal reasoning and learning. The authors present a look at their study in progress aimed at assessing their theory. According to the model of play they outline, imaginative play serves as an engine of learning. Such play arises out of the human capacity for causal cognition and feeds back to help develop causal-reasoning skills. (Contains 1 figure.)