Once your baby is 4 to 5 months old, she'll be fascinated as you pop up from behind your hands. By 6 to 8 months, she'll be playing along, hiding and then giggling when she makes eye contact.
Why it's so great: Your baby is learning a vital lesson in object permanence. "An infant doesn't realize that things continue to exist after they're out of sight," says Charles Nelson, PhD, professor of pediatrics at Harvard Medical School. "She enjoys the surprise element. You disappear and reappear, which is thrilling to her and teaches her that things come back, which will help her deal with separation anxiety later on."
How to make it even better: Hold a blanket between the two of you. This adds a lesson in body control -- she'll have to plan to remove the blanket, reach for it, and pull it away.