The present study investigated the development of possibility-judgment strategies between the ages of 4 and 8. In Experiment 1, 48 children and 16 adults were asked whether a variety of extraordinary events could or could not occur in real life. Although children of all ages denied the possibility of events that adults also judged impossible, children frequently denied the possibility of events that adults judged improbable but not impossible. Three additional experiments varied the manner in which possibility judgments were elicited and confirmed the robustness of preschoolers' tendency to judge improbable events impossible. Overall, it is argued that children initially mistake their inability to imagine circumstances that would allow an event to occur for evidence that no such circumstances exist.