How to tell if you're a boy

My daughter told me she asked D how he knows he's a boy. I'm not sure what sort of an answer she was expecting—but D looked at her as if she were the single most thick-headed individual on the planet, and said, "Because I have a Kipah!"
When they were at our house, she asked K the same question. He said, "Because I don't have a sheitel or any clothes that are pink."