Best jokes from Primary school.

This is the thread for all those jokes you remember from Primary. Think back to those simpler days, when all a joke needed was a reference to male genitalia to make everyone that heard it erupt into giggles. The more immature the better. To start:
My uncle Billy,
Had a very big willy,
So he showed it to the lady next door.
She thought it was a snake,
So she cut it with a rake,
And now it's only three foot four.