Subtle.
A man is in bed in a private hospital ward waiting to be made ready for an operation. There's a knock at the door and he calls, "Come in!"
A pretty young nurse enters and says, "I'm going to prep you for your operation. When I'm finished, if you have any questions I'll be happy to answer them."
She stripped him, shaved his private parts, took his temperature with an anal thermometer, then dressed him in one of those dreadful hospital gowns that opens down the back before placing having him lie on a gurney to be wheeled to the theatre. "Now," she asked, "do you have any questions."
"Just one."
"Oh, good. What is it?"
"Why did you knock?"