A retired sailor puts on his old uniform and heads for the docks once more for old times sake. He engages a seasoned prostitute and takes her up to a room, paying her fee in advance. He’s soon going at it as well as he can for a man of his age, but needing some reassurance, he asks, “How am I doing, darling? Am I up to speed?”
The prostitute replies, “Well old sailor, I'd say you’re doing about three knots.”
“Three knots!” he asks, “I've sailed the seven seas, but forgive me, what’s that supposed to mean?”
She says, “You’re knot hard, you’re knot in, and you’re knot getting you’re money back!”