A baby's first laugh is usually reserved for the people who first made him smile: Mommy and Daddy. It begins with parents making faces and funny noises to get the baby excited and interested, notes Doris Bergen, PhD, a professor of educational psychology at Miami University in Oxford, Ohio. A baby's relationship to the adult doing the funny stuff matters as much as the physical sensations and funny noises; he'll laugh and play only if he feels secure.
A little later, what makes an infant giggle is primarily physical and feels pleasurable: blowing raspberries on his belly, tickling his feet, picking him up and flying him gently through space.
At about 4 months, a baby begins to laugh at things he can see and hear. He'll delight in nonsense humor -- an exaggeration of things he typically experiences, such as faces with wide-open mouths and big eyes and wacky sounds such as toots and trills.
Your baby's giggles mean he's having a good time, but he doesn't yet have a true sense of humor. He'll begin to develop one within the next six months, when he develops the cognitive ability to find an idea funny.