MATH PUZZLE: Can you replace...

A little girl and boy are fighting about the differences between the sexes, and which one is better. Finally, the boy drops his pants and says, “Here’s something I have that you’ll never have!” The little girl is pretty upset by this, since it is clearly true, and runs home crying. A while later, she comes running back with a smile on her face. She drops her pants and says, “My mommy says that with one of these, I can have as many of those as I want!”