An old man, vacationing in Las Vegas, decides to visit a brothel, as it has been a long time since he's had any. After paying the madam, he picks out a cute little blonde girl, and they go upstairs.

After the preliminaries, he climbs on and starts humping away, and hollers out, "How am I doing, honey?" She replies, "About three nots."

He says, "Three knots? What does that mean?"

She says, "You're not in, you're not hard, and you're not getting your money back!"