Sex Education

One day in class the teacher has sex education. On the black board she draws a penis then asks the class if any of them knows what it is. In the back of the room, Dirty Johnny stands and says "That's a penis, and my father has two of them."
The teacher looks surprised and asks, "What do you mean, two?"
Dirty Johnny responds, "A little one to pee, and a big one to brush the baby sitter's teeth."