On Dec 9, three Chinese astronauts gave a science lecture（讲课）400 kilometers above Earth！Zhai Zhigang, Wang Yaping and Ye Guangfu, who are working aboard the Tiangong space station, showed millions of students a glimpse（一瞥）of their life there. They also did some fun science experiments（实验）. Let's take a look at some of them.
Use it or lose it
The human body grows with the pull of Earth's gravity（重力）. But in space, our bones and muscles（肌肉）don't need to sustain our body's weight. This will cause bone and muscle loss. NASA says that astronauts can lose up to 20 percent of their muscles after just a couple of weeks. That is dangerous because they will be too walk to work.
Astronauts have different ways to prevent this. During the lecture, Zhai introduced a special uniform Ye was wearing. It's called a "penguin suit". There are many elastic bands(弹力带) inside the suit. Wearing it, astronauts use their muscles to stretch the bands. This keeps their muscles strong.
Wang also showed students how she walked on a treadmill(跑步机) and rode a bicycle. Astronaut exercise with these tools every day to prevent muscle loss.
Space water is different
During another experiment, Wang made a water film（膜）. Then she added more water drops onto the film to make it a water ball. This can never happen on Earth. Why？
We need to first understand the surface tension（表面张力）of water. It is like a "skin" of water. The "skin" is there because water molecules（分子）are drawn to each other. Most of the time, gravity is too strong and it breaks the "skin". But with zero gravity in space, the "skin" stays strong and the water ball will not break.
Space water is different in many other ways from that on Earth. For example, water will not flow downward if you try to pour it into a cup. That's why astronauts usually drink water through straws（吸管）in sealed（密封的）bags. If astronauts cry, the tears will stay on their eyeballs instead of falling down. They have to wipe them away！
