Jack worked in a shop that sold clocks. He was always telling Harry to buy a new clock. But Harry, who lived next door to Jack, said he didn’t need one. “ Everyone needs a clock,” Jack said. “How do you know when it’s time to get up?” “My landlord Mr. Smith turns on his radio at seven o’clock and listens to the news,” Harry said. “That’s my morning call.” “Ok. But how do you know when to go to work?” “By the time I have eaten my breakfast, it’s eight o’clock, time to leave for the office. Then I walk there. When I arrive at my office, it’s nice o’clock. That’s the time I start work.” “Ok. But how do you know when it’s time to go home?” “The factory bell rings ” Harry told him. “But how do you know when it’s time to go to bed?” “The television programs come to end.” By now Jack was really angry 。 “Ok,” he shouted. “Now tell me what would happen if you woke up in the middle of the night and wanted to know the time.” “That’s easy,” Harry said, “I would knock heavily on your wall. Then you would shout at me, ‘What are you doing knocking on my wall at three o’clock in the morning?’”