Prior research has shown that children hold a belief in causal determinism - the belief that all events are caused – by 4 years of age. In this study we investigate the developmental origins of this belief. We showed toddlers (24 months) a spontaneous or explained novel physical outcome (a toy that lit up either spontaneously or upon contact from an experimenter) and then showed them an additional candidate cause (pressing a button) while obscuring the outcome. We asked whether toddlers inferred that the two components (the button and the outcome) were causally linked. We found that toddlers represented the candidate cause as the cause of the novel outcome only when the event spontaneously occurred (Experiments 1-2), and that children spontaneously searched for plausible causes of unexplained outcomes (Experiment 3). These results suggest that toddlers, like older children, believe physical events have causes, and that this belief supports exploration and discovery.