oldie but still funny
A guy goes to the supermarket and notices a very attractive woman waving at him.
She says, 'Hello.'
He's rather taken aback because he can't place where he knows her from.
So he asks, '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.
So he asks, 'Are you the stripper from the bachelor party that I made love to on the pool table, with all my buddies watching, while your girl friend whipped my butt with wet celery?'
She looks into his eyes and says calmly, 'No, I'm your son's teacher.'