The challenge is for you to come up with a more witty/creative/original completion to a given sentence then me (and the roughly 40 other people who contribute) thereby "one up-ing me".
Example (Yesterday's winner, which just so happened to be posted by none other then myself.):
"He was like an elevator: never to be trusted in cases of emergency."
The registration is perfectly harmless (it doesn't even require an email address). Once you have a log-in you are able to post, as well as vote for the entries you think are best (just reading the entries can be entertaining in and of itself).
Every morning at 7:00 (eastern) a new sentence is posted along with the previous days winner. You have until 7:00 in the evening to post a reply and until midnight to vote.
I'm putting a link on my blog under "Site I Visit", if you are ever looking for a productive mentally stimulating way to burn some time.
...and it's a lot more fun then Sudoku.