Chalk it up to the cave man instinct, but boys play with their penis practically from birth. (And those baby erections? Whoa.) What's even more shocking: As soon as your little man can speak, the mine-is-bigger-than-your talk starts. Around age 2.5, you'll start to hear things like: "Dada, your penis is huge!" "My penis is tiny!" "My penis is bigger than his." "Mama doesn’t have a penis! How do you pee?" Think of it this way: At least they're learning some basic mathematical concepts.