The boss of a big company had a tough choice to make. He had to fire somebody. He had it narrowed down to one of two people, Debra or Jack. It was an impossible decision, they were both super workers. Rather than flip a coin, he decided he would fire the first one who used the water cooler the next morning.
Debra came in the next morning with a horrible hang-over after partying all night. She went to the cooler to take an aspirin. The Boss approached her and said: "Debra, I've never done this before, but I have to lay you or Jack off."
She replies, "Could you jack off? I have a headache."
(It's a really bad joke, but try explaining it to your students without getting fired!)