Infants as young as 3 months old love to watch other children. An older sibling saying “Boo” in the baby’s face or jumping up and down will quite likely elicit the baby’s first genuine laugh. During the course of the first year, the baby will engage in reciprocal play with an older sibling or peer, knocking down a tower that the sibling has built or creeping after a wheel toy that the peer is pulling around the room.