Every day a 4th grade boy walks home from school past a 4th grade girl's house. One day he he stops to taunt the little girl. He holds up the football and says "See this football? Football is a boys game and girls can't have one!"
The little girl runs in the house crying and tells her mother about the encounter. She runs out and buys the girl a football. The next day the boy is riding home on his bike, and the girl shows him the football, yelling "Nah na nah na nah".
The little boy gets mad and points to his bike. "See this bike? This is a boys bike, and girls can't have them!"
Next day, the boy comes by and the little girl is riding a new boys bike. Now he is really mad. So he drops his pants, points at his private parts, and says "You see THIS? Only BOYS have these and your mother can't go buy you one!"
The next day as he passes the house he asks the little girl "Well, what do you have to say NOW?"
So she pulls up her dress, points to her private part and says "My mother told me that as long as I have one of these, I can have as many of THOSE as I want

Old joke but still funny