Very young children have remarkably sophisticated causal knowledge about the world, yet relatively little is known about the process of causal learning. In this paper we provide a Bayesian model of how the interaction of prior theories and evidence can lead to ambiguity in competing causal hypotheses; we suggest that children seek to resolve such ambiguities through active exploration. In Experiment 1, we look at the model with respect to children's causal judgments. In Experiments 2 and 3, we show that children selectively engage in exploration when evidence is formally ambiguous with respect to their prior theories. We suggest that children's play is rational with respect to this model and that children's active exploration of causal ambiguities might generate evidence that could support theory formation and theory change.