There was a big pile of bricks on the top of a tall building and a man had to bring them down to the ground. He had a long rope which went over a pulley (滑轮) at the roof of the building. The other end was fastened to a big box. First he pulled the box up to the top of the building, and fastened the end of the rope so that the box could not come down. Then he climbed up the ladder and filled the box with bricks. Next he climbed down the ladder and untied the rope. Unfortunately the box of bricks was heavier than the man, and as a result he was pulled up by the rope. Half-way up, the box of bricks hit him as it was coming down. When he reached the top, his head hit the pulley. The box of bricks hit the ground and broke. As a result the bricks fell out. The box was then lighter than the man and consequently he began to come down and the box began to go up. Half-way up, the box hit the man. The man still held on to the rope and the box therefore continued to go up. The man then reached the ground. Then he let to the rope to rub his head. The box, of course, then fell down and hit the man on the head again. As a result, an ambulance came and took him to hospital. Why did the man fasten the end of the rope before he climbed up the ladder?