Bob and his girlfriend Alice were having dinner with Charlie and his girlfriend Dora. They were having great fun and conversation and started feeling amorous toward each other, so they traveled to the nearest hotel with the intention of getting some action. Then Bob suggested, "You know, I'm feeling particularly naughty tonight, Alice, wouldn't it be exciting if we traded partners just for the night?" This excited Alice so much, and as it turned out, Charlie and Dora were in a similar mood and heartily endorsed the idea.
The next morning, after a passionate night of lovemaking, Bob turned to his new partner and said, "Well, Charlie, that certainly was exciting. But best we get up and see how the girls are doing."