It’s a joke that hasn’t become a joke yet because the world is about to end. Soon, but not now.
The rain hasn’t stopped. All the water is outside but a fish walks onto the bus. Our breath fogs the windows and we wonder if the fish will suffocate before or after we reach our stop. We hope it’s after.
A fish walks onto the bus. The joke is waiting for the world to end. The fish opens its mouth and works up a laugh that sounds like marbles in a sack of water. We start working on our punchlines.