Well, this is the first actual joke I remember my mother teaching me (as opposed to knock-knock jokes and conundrums), when I was about three or four years old.

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Man goes into the doctor's office, and the doctor asks what's wrong.

"Well, I feel fine," the man said, but when I woke up this morning, I noticed there was a screw in my belly button, and don't know how it got there, and I'm a bit concerned."

"Ah, yes. I see," the doctor said, and had the man undress and perch on the side of the examination table. Sure enough, there's a big fat shiny Phillips-head screw right where the man's belly button should be.

"Hm," the doctor said, after a moment's examination. "There doesn't seem to be any injury. Let's just see if we can get this thing out." And he digs around in his cabinets until he finds a Phillips-head screwdriver. And he starts unscrewing, and the screw gets looser and looser.

"Does that hurt at all?" the doctor asked.

"No," the man said. "It feels a bit odd, but it doesn't hurt."

So the doctor keeps going, and soon the skew is out completely.

"Thank you, doctor," the man says. That's quite a relief.

He stood up to get dressed.

...and his butt fell off.

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I don't know which is more disturbing: the joke, or that my mother taught the joke to her four-year old daughter. ;-)