Understanding how responses become prepotent is essential for understanding when inhibitory control is needed in everyday behavior. The authors investigated the conditions under which manual actions became prepotent in a go/no-go task. Children had to open boxes that contained stickers on go trials and leave shut boxes that were empty on no-go trials. In Experiment 1 (n = 40, mean age = 3.6 years), the authors obtained evidence consistent with this task requiring inhibitory control. Results of Experiment 2 (n = 40, mean age = 3.7 years) suggested that box opening was prepotent because (a) opening is the habitual action associated with boxes and (b) children planned to open boxes on go trials of the task. Experiment 3 (n = 96, mean age = 3.5 years) showed that even empty boxes elicited the same errors and that delaying responding reduced errors even though the delay occurred before the cue that indicated the correct response (contrary to a rule reflection account). Because the delay occurred after box presentation, performance was consistent with a transient activation account. Delay training might benefit children with weak inhibition.