Get ready for your baby to start putting everything in his mouth! By 4 months, your baby's mouth is his prime tool for exploration, and he'll likely become fascinated by the sounds it can make. At this age, your baby will become a lot more social and interactive as well as more physically active, so play will become more important than ever.
Go ahead and let your baby explore things with his mouth (as odd as it may seem to you) and buy him toys designed for gnawing (these will also be very soothing as your baby starts teething). Just be sure to keep his toys clean and make sure they're not a choking hazard (they shouldn't be able to fit through a roll of toilet paper).
Rolling a ball to your baby will help strengthen his muscles and improve coordination, and sending it slightly to his right or left during tummy time may help your baby learn to roll over, a major milestone on the horizon these days. Give your baby plenty of play time on his back and his belly so he can explore his world from all angles.
It's also a good idea to place a few crib-safe toys in your baby's crib or play yard so he can use them to entertain himself when you are otherwise occupied. Give him toys that make different sounds in response to different actions and watch how fast he learns that rattles are for shaking and squeaky toys should be squeezed. --Jennifer Halpern, PhD
Originally published in the November 1999 issue of Parents magazine. Updated 2009.