1 1/2 to 2 months
What parents could possibly forget the first time their infant intentionally flashes a gummy grin? These "social smiles" (as opposed to the fleeting grimaces infants make when they're gassy) are heartwarming, but they're also a sign that the parts of your baby's brain that control eyesight and muscle movements are booting up.
A smile also shows off your child's newfound social savvy. She's starting to understand the positive emotions a happy face conveys—and that making one can get her plenty of attention. Encourage her by smiling at her as often as you can and, of course, acting surprised and pleased when she smiles back.
4 to 6 months
Laughter is instinctive, but it's also something your baby has to learn. By this age, he's heard other people laughing many times, and he's gained enough control over his vocal cords to realize it's something he can do too.
Don't be surprised if he laughs like Flipper at first, making a staccato "heh-heh-heh" and then a high-pitched squeak as he inhales. The reason: His larynx is still small and floppy, and he doesn't have good control over it. Savor this ultra-cute phase while it lasts. By his first birthday, his laugh will be sounding a lot more like your own.
4 to 6 months
Babies love to do the Bronx cheer, even when your favorite baseball team doesn't lose the playoffs. What gives?
It's part of learning how to talk. Your child is experimenting with different ways to move her lips and tongue: Sooner or later, she'll probably make a raspberry—and love it. (About 25 to 33 percent of all babies never do, though, and just go straight to babbling.) Go ahead and egg on your little blowhard, maybe with some raspberries of your own. It will encourage her to try other sounds, which is great practice for forming consonants, vowels, and—eventually—words.
Eating His Feet
4 to 8 months
Sooner or later, you'll probably find your baby happily sucking on his toes. Yes, this is strange, but it's still an important milestone. His hands aren't very coordinated yet, but he's jonesing to learn more about the objects around him. So he explores things by putting them in his mouth—including his own feet, once he's found them.
Besides promoting body awareness, toe-sucking is very satisfying and soothing to your little one. So don't discourage it; this phase generally passes on its own. (And don't be concerned if he never does this trick, by the way—not all babies do.)