There was a famous church in London. It was 110 meters high. The center of the church was like a bell. The ceiling of the church was painted by a famous artist. A platform （平台）was built so that the artist could put up the ladder on it and paint the ceiling. The platform was about 70 meters over the ground.
    One day the artist was working hard and had nearly finished one popular corner of the ceiling. Also on the platform was his assistant, who was mixing paints. The artist stepped back so that he could see how his work was progressing. To have a better look at his painting, he stepped back again. Suddenly, his assistant shouted, picked up a small bowl of paint and started to paint the artist’s work on the top. The artist was very angry and rushed forwards to stop him.
    “What do you think you are doing?” he shouted angrily. “Are you mad?” His assistant replied, “I saw you walking backwards to have a better look at your painting. But you didn't notice that you had reached the very edge of the platform. You were in great danger. I wanted to make you move forwards. If I had not made you run forwards, you would have fallen backwards off the edge of the platform.