At around 4 months, your baby may surprise you with a small chuckle or even a full-out belly laugh.
In the beginning, chuckles and laughs are a physical response to something you’re doing like kissing quickly, tickling or blowing air in his belly.
Later on, when your baby laughs at something external, like the look on your face in response to any of his actions.
it means he’s starting to develop her own sense of humor, and he clearly finds you amusing. Encouraging his newfound funny bone is easy: Just keep doing silly things.