A guy is at the supermarket when he notices that the rather good looking blonde behind him has just
raised her hand and smiled hello to him.

He is rather taken back that such a looker would be waving to him, and
although familiar, he can't place where he might know her from, so he
says "Sorry, do you know me?"
She replies "I may be mistaken, but I think you might be the father of one of my children."

His mind shoots back to the one and only time that he had been unfaithful.
"Christ!" he says, "are you that strip-o-gram on my stag night that I
screwed on the pool table in front of all my buddies while your friend whipped me with some wet celery and stuck a cucumber up my ass?"

"No" she replies, "I'm your son's English teacher."