So here's a dirty joke, but it's for a good cause:
A guy goes to his doctor and says, "Doctor, I've got this problem with my knee."
"Well," says the doctor, "you've got to stop masturbating."
"What? Why?" asks the guy.
"So I can examine you," says the doctor.
As many times as I've heard it and told it, this joke never fails to induce hilarity. It makes excellent use of humor's secret weapon, the unexpected. But in this case, there seems to be something deeper going on. There is another level of shock that is integral to this joke's effect and it relies on the same device as, say, the move the Sixth Sense. At the end, you are given a piece of information that undermines everything you understood before. It collapses the entire world you thought you were inhabiting and makes you realize that the story you were hearing was actually something completely different than you thought. It's a powerful device, though one that has admittedly been clumsily employed and overused in the last ten to twenty years of popular storytelling. But this joke seemed to me a particularly elegant and illustrative example of it. In just four lines a world is built, collapsed and reconstructed to immediate and extreme emotional effect. What more could you want from a story?