She says hi, and he's rather taken back because he can't place where he recognizes her from.  

So the guy says, "Do you know me?"

The woman replies, "I think you're the father of one of my kids."

Now his mind travels back to the only time he cheated on his wife and says, "Holy shit!  Are you the prostitute from Vegas that I had sex with at halftime of the basketball game?  We were doing it on the coffee table with all my buddies watching while your partner tickled my ass with a pineapple?"

She looks into his eyes and says calmly, "No, I'm your son's teacher."