A man's walking home late at night when he sees a woman in the shadows. "Twenty bucks," she says. 
He's never been with a prostitute before, but he decides what the hell. 
They're going at it for minute when all of sudden a light flashes on them - it's a policeman.

"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, "to tell the truth neither did I until you shined that light on her face."