A ten-year-old kid asks his father, "What's a penis?"
The father whips his own out and says, "This is a penis, son. It's a perfect penis."
The next day, the ten-year-old is playing with a friend who asks if he knows what a penis is.
The kid whips his out and says, "This is a penis. And if it were two inches shorter, it would be a perfect penis."
