One day, Tim mathematic teacher looked at his homework and saw that he had got all his sums right, The teacher was very pleased-and rather surprised, He called Tim to his desk and said to him, "You got all your homework right this time, Tim. What happened? Did your father help you?" http://www.xue90.cn来自《幽默学英语》网
Usually Tim’s father did help him with his homework, but the evening before this, he had not been able to, because he had not been at home, so Tim answered, "No, sir. He was busy last night, so I had to do it all myself."