From http://www.babycenter.com :

“Want a foolproof way to make your baby laugh? Mimic his facial expressions and sounds. Not only will he be tickled pink, but he’ll also learn something about communication, since imitation and repetition are key ingredients in early brain development. If he hears you say the same words often enough, he may even begin to make some almost recognizable noises, including ones that sound suspiciously like “Ma-ma” and “Da-da”….”