Two impossible events: At about 5 months of age babies were surprised by the physically impossible sequence in A—but they did not look surprised by the event in B till about age one. The bottom line: Infants understand the physical world far earlier than Piaget believed, but this knowledge occurs gradually.

Adapted from: Baillargeon, 1993; Baillargeon & DeVos, 1991; Baillargeon & Graber, 1987.