Unfaithful Dad

A guy goes to the supermarket and notices a beautiful blond woman wave at him and say 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 kids."
Now his mind travels back to the only time he has ever been unfaithful to his wife and he 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?"
"No," she calmly says, "I'm your son's math teacher."