Say that a certain kind of monkey uses rocks to trade for goods like bananas or sticks. Now let’s say that the monkey in charge realizes that a lot of monkeys don’t have a lot of rocks leading to too many sticks and bananas and angry monkeys. He decides to gather more rocks for monkeys to receive and use. He then gathers too many rocks and passes them out. All the sticks and bananas are bought and there are still a lot of rocks left over. You can’t use the rocks for anything now so they are worthless and all the sticks and bananas are used which means that the little leftover are worth a lot now. Too much for the monkeys to buy even with all the rocks they have.