Once a traveler wanted to go across a desert. He had walked in the desert for three days without any water. He was exhausted (筋疲力尽的) from walking. When he felt hopeless, he suddenly found a small house. He pulled his tired body inside. This was a small airless house and in the corner, he discovered a water pump (抽水机).

He was so excited and went forward to draw (抽取) water, but no water came out no matter how hard he tried. Too tired to do anything more, he fell to the ground. Then he saw a small bottle. On the bottle was a note, which reads," You must pour the water from the bottle into the pump before you draw water! Don't forget, before you leave, refill the bottle with water! "He opened the bottle and found it was filled with water! He was so happy.

At that moment he thought, "It might be a little selfish(自私的), but as long as I drink the water in the bottle, I can be sure to leave this rom alive! However, if I do as the note says and pour the only water into the pump and no water comes out, I will die of thirst. Should I take the risk or not?" It was five minutes later that he chose to pour the only water into the pump and began to draw water. After a minute, water started to come out! He made the right choice.

After he had drunk enough water, he refilled the bottle. He added the following words after the note: Before you get something, you should first lean to pay.