In the town, there is a poor farmer and he has a kind wife. They only have a farm, a horse and a cat. One day, the farmer talks to his wife, “We can exchange(交换) the horse with other goods on tomorrow’s market.” His wife answers, “Yes, we can. Because we needn’t use the horse.”
The next day, the farmer takes the horse to the market. He finds a good cow and he thinks, “We can drink milk if we have a cow.” So the farmer exchanges the horse with a cow. Then he wants to go home. A few minutes later, he sees a fat duck and wants to eat it. So he talks to the owner of the duck. The duck’s owner feels happy because the cow is more expensive than the duck. So the farmer exchanges again. Finally, he feels very happy and goes home.