Can infants perceive stability when a supported box is put on a supporting box in a balanced position? In Experiment 1, 41 infants saw three events. In a stable event, the supported box was put on a wide supporting box in a balanced position. In an unstable event, the supported box was put on a narrow supporting box in a balanced position. In an impossible event, the supported box was put on the extreme end of a wide supporting box. Infants 4 to 6.5 months old looked equally at all three events. Infants 6.5 to 10 months old looked slightly longer at the impossible event than at the other events. Infants 10 to 13 months old looked reliably longer at the unstable and impossible events than at the stable event. The results of Experiment 2 indicate that these differences in looking times did not come from differences in stimulus configurations between the events. These results suggest that infants above 10 months old are sensitive to stability of support relations.