One morning, a little mouse went out to find something to eat. He was careless and met a fierce(凶猛的) lion. The lion caught the mouse. The lion wanted to eat him. 

"Please let me go, Mr Lion," said the mouse. "One day, I will help you. "

The lion laughed and thought, "How could a little mouse help a big lion? ""Well, "he said, "I will let you go. But you must walk more carefully. " The mouse was very thankful. "Thank you, Mr Lion. You are very kind. I will repay (报答) you one day, "the little mouse said.

The next week, the mouse was looking for something to eat again. He saw the lion under a tree. But the lion was tied tightly(紧系着) in a strong rope. He could not get out. 

"I will help you, " said the mouse. His teeth were sharp. Then he bit one rope, and then another. Soon the lion was free. He was very happy. "Thank you, Mr Mouse, " he said. Later, they became good friends.