If I had a dollar for every time somebody asked me "Wow, what'd you do to your leg?" or some such variation, I'd have, like, a zillion dollars. And I've only been doing this for like a year! At first I would tell people the truth, but then I decided that the truth is BORING and I could probably do a lot better. That's when I came up with this game. Basically, every time someone asks you what happened, you make up a much more interesting story right there on the spot -- the more obviously ridiculous it is, the better (because LYING is no fun, but creativity totally is). Example: "I was robbing a bank and it went bad, but we stole enough money to hire a good lawyer," "I was a bad guy in a Jet Li movie," "I sprained it doing competitive Bonsai," "I'm actually a superhero and this is part of my cover identity," etc.
Then I figured, it translates pretty well to a forum game, too. I know not everybody has the whole traumatic-injury thing going on, but this one time maybe it's fun to pretend. Give it a shot!
What happened to your knee?