Gravity
重力
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R,Q]qskEUsg8TRLMQL)。这就叫做重力lH9O%j%xA8Y^CBAZaBP。
Gravity pulls from the center of the Earth.
地心引力来自于地球中心-2EV8~_q0yO&-tm。
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*9WQN91)W(^@。
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ZXzKnUH[KZ@Rx[kb。
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4wvxM2.q^OUH4CR]!16,。