A guy goes to the supermarket and notices a beautiful blond woman wave at him and call out "Hello". He's a little taken aback because he just can't place where he knows her from. So he says "Do we know each other?" To which she replies "Yes, I think you're the father of one of my kids."

His mind races back to the only time he'd been unfaithful to his wife and says "Oh, my God, are you the stripper from my bachelor party that I laid on the pool table with all my buddies watching while your girlfriend spanked my butt with a celery stick????"

She looked calmly into his eyes and said "No, I'm your son's math teacher."