One afternoon a board in the market flipped over: TUNA PRICE ▲. Out in town, tuna had suddenly become very popular.

Inside the kitchen, though, the pool was still selling tuna at the old price. So clever traders hurried in, bought the cheap tuna from the pool with rice, and sold it in town for more. They kept going until the pool’s price matched the town’s.

When it was over, Maki peered into the bowls. There was less tuna than before, and more rice. “Uh-oh,” said Maki. “I think we got a bit… rearranged.”

“Let’s say you’d kept your tuna and rice at home,” said Uni, who had seen this many times before. “When tuna got pricier, all of your tuna would be worth more.”

“But in the pool, traders took some of your tuna away and paid you rice. So you own less of the thing that went up, and more of the thing that didn’t.” Uni shrugged. “You still made money, probably. Just less than if you’d done nothing.”

“That’s called impermanent loss,” Uni went on. “Impermanent, because if tuna goes back to its old price, the bowls go back too, and the loss disappears. And remember the tips? Swap fees keep landing in the pool the whole time. Sometimes they’re worth more than the loss.”

Move the price of tuna up or down and compare two cupboards: one where you just held your tuna and rice, and one where you put them in the pool.

Notice that it doesn’t matter which way the price goes. Up or down, the further it moves, the bigger the gap. Then add some fees and see whether they win.

Maki took a long, deep breath (rolls can do that) and decided that, all things considered, it was still worth bringing bowls. Especially now that the kitchen was about to go on a very long voyage.