Peekaboo!

My eldest daughter loves to make her baby sister laughes by playing peekaboo. This game has never failed my youngest daughter to laugh and squel loudly. Why? Because it mimics what she is just coming to understand about the world: that objects and people still exist even when he can't see them (a jack-in-the-box is fun right now, too.). That's why she finds quick appearances and disappearances amusing.

Understanding this concept, also known as object permanence, is an important milestone, and signals that your baby is making leaps in cognitive development — both his memory and his ability to think abstractly are growing.