What happens if you drop a book? 如果你丢一本书会发生什么？

　　Absolutely, it will fall down to the floor. This is called gravity. 很显然，书会掉到地上。这就叫做重力。

　　Gravity pulls from the center of the Earth. 地心引力来自于地球中心。

　　So things fall down to the ground because of gravity. 所以，物体落在地面上都是因为重力。

　　Do you know what would happen without gravity? 你知道没有重力会怎么样吗？

　　Everything would float around in the air. 所有物体都会漂浮在空中。

　　When you fly high away from the Earth into space, you will float around. 当你飞离地球到太空时，你就会漂浮起来。