Open Question: Why does my knee lock up?

Sometimes when I bend my knee in for whatever reason, it locks in place. What happens is I'll bend my knee up (I've noticed it USUALLY happens when I bend in quickly, although since it happens infrequently, I might be wrong), I'll feel this sort of popping sensation in my knee, and then the underside of my knee will feel a little weird (sorry I can't describe it any better than that).