A guy goes to a supermarket and notices a beautiful blonde who waves at him and says hello. He's rather taken aback, because he can't place where he knows her from.

So he says "do you know me?".

To which she replies "I think you're the father of one of my children."

He thinks back to the only time he has ever been unfaithful and says... "My God, are you the stripper from my bachelor party that I laid on the pool table with all my buddies watching, while your partner whipped my butt with wet celery ?"

Suddenly looking very uncomfortable, she said "No, I'm your son's math teacher.