A teacher and a farmer are sitting on a plane. They both feel bored.

The teacher says to the farmer， “Lets ask each other a riddle（谜语）. If one of us cant answer it， he should give the other $100.”

“Thats not fair（公平的）！ You know more than me，” says the farmer.

The teacher says， “OK. If you dont know the answer， you pay me $50. If I dont know， I pay you $100. Do you agree？”

“Yes， lets play. Ill go first.”

The farmer says， “Here is the riddle. What has three legs when it walks， but only two legs when it flies？”

The teacher thinks for a while and gives up（放棄）. He pays the farmer $100 and says， “Tell me， what is the answer？”

“I dont know， so I will pay you $50.”