A man's walking home late at night when he sees a woman in the shadows.
"Twenty bucks," she says.
He'd never been with a hooker before, but he decides what the hell it's only twenty bucks. They're 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," the man answers indignantly.
"Oh, I'm sorry," says the cop, "I didn't know."
"Well," said the man, "neither did I until you shined that light in her face."