A man is returning home from work when a mysterious voice in a dark alley whispers, "20 bucks?"
The man decides to take the mystery woman up on the offer. They've been going at it for a few minutes when a police officer shines a flashlight on them, demanding to know what was going on.
The man irritably says, "Do you mind? I'm making love to my wife!"
The police officer apologizes for interrupting, stating that he did not know.
The man replies, "I didn't know either, until you shined your flashlight on us."