Two couples had gone away for the weekend. The two guys, Jack and Bill, have decided to try to persuade their wives to do a bit of partner swapping for the night. After several drinks that night they succeed.
Jack knows it's that time of the month for his wife and the thought of Bill not knowing this makes him smile. The guys agreed that when they sit around the breakfast table the following morning, they will tap their teaspoons on the side of their coffee mug the number of times that they did it with each other's wives.
The next morning they are all at the breakfast table, slightly hung over and quite uncomfortable, when Jack proudly taps his teaspoon 3 times against his coffee mug. After a brief moment of thinking, Bill takes his teaspoon and taps it once on the strawberry jam and 3 times on the peanut butter!