Joe was walking home late at night, through the park, and sees a woman in the shadows. "Twenty dollars," she whispers.
He's never been with a hooker before, so he figures, "What the hell, it's only twenty bucks..."
So they hide in the bushes and are going at it for a minute when all of a sudden a light flashes on them. It's a police officer.
"What's going on here people?" asks the officer.
"I'm making love to my wife," Joe answers indignantly.
"Oh, I'm sorry" says the cop, "I didn't know."
"Well," Joe says, "I didn't know either until you flashed that damn light in her face."