Astronauts (航天员) Gui Haichao (L) and Zhu Yangzhu were playing “ping-pong” in the Tiangong space station on Sept 21. They gave a science lesson to students on Earth. First, they made a small water ball. Then they put towels (毛巾) on the outside of the ping-pong bats and hit (击打) the ball. The water ball traveled in the air just like a ping-pong ball.