I Set My Clock By Your Whistle

Every day a man used to walk by a jewelry store, stop and set his watch by the big clock in the window. One day the jeweler happened to be standing in his doorway. He greeted the man in a friendly way and said; "I see you set your watch by my clock. What kind of work do you do that demands such correct time each day?" "I'm the watchman at the plant down the street," said the man. "My job is to blow the five o'clock whistle." The jeweler was startled. "But...you can't do that," he blurted out. "I set my clock by your whistle!"