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You hide a toy behind your back, and your baby peeks around your waist to find it -- it's a simple game that's fun and teaches object permanence, the realization that items still exist even when they can't be seen. Researchers at the University of Washington, in Seattle, found that 9-month-olds could find a toy that was partially hidden under a cloth, and by 10 months, the majority could find a fully hidden toy if sound clues, like a beep, were given. Offer your baby lots of opportunities to seek and -- success! -- find.

Rotate toys. Swap containers of toys every week or so. "Babies stop exploring when they think they know everything that's in the pile," says study author M. Keith Moore. But if your little player hasn't seen those stacking rings for a while, he'll enjoy them -- and learn from them -- that much more.

Resist playing cleanup wizard. "Instead of putting away the toys every night, leave some playthings on the floor where your baby played with them, so he can learn the next day, 'Oh, it's still there,'" says Moore. He'll realize that an object doesn't move unless he does the hefting.

Spice up your peekaboo. Hide a stuffed animal under a blanket and move it all around. Your baby will love watching the toy try to escape, and squeal when it does.