Dirty Johnny and his friends were enrolled in sex education. During the lecture, the teacher kept referring to a "penis". It was "penis" this and "penis" that. The class behaved as if it understood.
Immediately following class, all the students gathered around Dirty Johnny. "Johnny, what's a penis?" Nobody had a clue. Dirty Johnny said he would ask his Dad when he got home.
When Dirty Johnny got home, his Dad was there. "Dad, what's a 'penis'?" His father unzips his pants, shows Johnny his dick and says, "This is a perfect penis." Johnny replied, "Thanks, Dad. I'll tell my friends tomorrow in school."
The next day, all the students gathered around. "What did your Dad say, Dirty Johnny?" Johnny promptly unzips his pants and shows all his classmates his dick. "This is a penis. And if it were two inches shorter, it would be the perfect penis!"