A boy goes into the bathroom and starts pleasuring himself, forgetting that he hasn't locked the door. In walks his Mum. "Erm... I... erm..." "That's OK", says his Mum, "but this is how you should do it" and she demonstrates ... anyhow, they get so carried away that before you know it, they're fully at it on the bathroom floor. When they finish, she says "you're even better than your father" to which the son says "yep, that's what my sister said too".