Two whales swimming around in the ocean come upon a ship that is hauling potatoes. The first whale swims underneath it, tips it over and eats everything, ship and all.
A little while later, they come up to another ship hauling potatoes. The whale capsizes that one, too, and eats everything onboard.
The third ship they spot is also hauling potatoes, and the whale upends it and eats everything.
"Why do you keep tipping over those ships full of potatoes and eating everything on board?" the other whale asks.
"I wish I hadn't done it," the first whale says, "but I just can't help myself once I start. You know how it is,
you can't eat just one potato ship."