In my experience, I've found that there are two general types of humor. The first is the blatantly obvious kind, like stand-up comedy, or some guy trying to speed past you on an icy sidewalk, then having his feet fly out from under him and totally eating it. You see it and your immediate response is to laugh, because you know you just saw something hilarious.
The second kind is more subtle. You still laugh, but you're not really sure why, because no matter how much you think about what you saw, you still don't understand what's so funny about it (think Frasier). Today's example is this story from ST, courtesy of John Hickey:
Dobbs, who was wearing a batting helmet at the time, had the helmet knocked off by the blow. But the fact that he was wearing the helmet seemed to protect him from any kind of serious injury.
Oh, wait. Now I know what's so funny.