I heard this one from my grandpa

There is a white kid an asian kid and a black kid playing together on a playground at school all in the second grade. The white kid asks if they want to play a game called whose penis is bigger and they all agree.

So the asian kid whips it out, the other kids say "not bad not bad"

Next the white kid whips his out, and they all agree that it's bigger than the asian kids

Finally, the black kid whips his out and the other two kids are in awe at how much bigger his is

So when the black kid gets home his mother asks him how his day went, and he told her about the game he played with his friends and how he won. So he asked her if it was because he was black. The mom looked at him and said "no, it's because your thirteen"