Why do babies take such a keen interest in other babies rather than teenagers or, say, a shoe on the floor? The truth is, until we can decipher baby babble, no one will know for sure. But there are some theories for what happens around the 6-month mark: Because they're mostly at the same low eye level, babies notice each other first, says Victoria Youcha, a child-development specialist at Zero to Three, a nonprofit early-childhood group in Washington, DC.

And it's their ever-expanding developmental abilities that make babies want to reach out and touch (and kiss, and hug) other little ones. "They are no longer run by their bodily functions," Youcha says. Their eyes are wide-open, they're taking in everything around them, and they're smiling. And who else is bound to be wearing that same grin but the kid in the next stroller?

So go ahead and let your baby coo to little strangers. He  -- and you  -- just might make a new friend.