Jokes. Where do they come from?

I've cracked a lot of jokes in my time, and as someone surrounded by funny people (at work and socially), the air around me is always full of them.

But I've never come up with a JOKE. You know, of the school yard, "tell me a joke" variety.

Steve-o: I came up with an honest to goodness joke once. It's pretty good too. You know the old, "What do you call a man with no arms and no legs in the water" jokes?
Me: Yeah. Bob.
Steve-o: Well I came up with, "What do you call the girlfriend of a man with no arms and no legs?" [wait for it] "Carrie."
Me: That's pretty good.
Steve-o: I know. And I've been waiting 20 years for it to catch on, for it to come back to me. I want to hear someone tell me that joke. But it never has.
Me: It definitely has that joke quality.
Steve-o: Pass it along. Tell people. Get it out there.