1 When you throw a ball up into the air as fast as you can, the ball reaches a point where it seems to pause for a moment, and then it comes down. 2 When a bullet ( 子弹 ) is shot straight up, it will travel much faster and higher than a ball, but it, too, will come down. 3 Whatever goes up must come down. 4 We have al-ways thought this to be true. 5 An airplane may climb to a height of seventeen miles and then travel far and long. 6 Yet it does not stay up forever. 7 Finally, like every-thing else, the plane must come down.