Saw this a couple of years ago on TV. Must be the best quip ever. But before I spill it see if you can get the gist of what it might be. I think it is perfect because it a) can only be told by one person on earth at a time, b) that person owns that joke until the next guy in line can say it, c) what allows the quip to change hands is sad but if it is your turn to own the quip just saying it says allot about your charachter.
Will tell all in a few hours....have fun. The joke may fall along the lines of "what colour was the bear" as far a figuring it out (if you know that one)