Yankee and the Red Sox

A teacher asks his students how many are Yankees fans and they all raise their hands except one lone boy.  So the teacher asks him why he isn't a Yankees fan.  The boy answers that he is a Red Sox fan.  "Why?" asks the teacher.  "Well," says the boy, my mother and my father are Red Sox fans and
I'm a Red Sox fan."  "What if your mother and father were idiots?" asks the teacher. "Then," says the boy, "I'd be a Yankees fan!"