This one's simple enough. Someone posts the first line for a limerick, then someone else has to post the next line, and so on. If you complete a limerick, you get to start a new one. Extra respect if it's actually funny.
Example limerick:
There once was a fellow from Leeds
Who swallowed a packet of seeds.
In less than an hour
His back was aflower,
And his hair was all covered in weeds.
I'm sure you can all do better than that, though.
OK, here's the first line:
There once was a dancer called Annie
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