I always thought you were supposed to make the joke yourself. Bronze Tiger's "story" might be funnier than mine, which might be funnier than Paste Pot Pete's, which might be funnier than Rev's, and so on.
No one's supposed to know or tell the story. Everyone's imagination is supposed to take hold and take you to places you've never imagined existed.
Exactly making up your own story is part of the fun.
Like I imagined the duck used to be a human and was a rival professor and Jenkins was jealous of him so he turned him into a duck. Never imagining that he would be shipwrecked and the duck would have the advantage.