Can you imagine astronauts' everyday life in space? They also need to eat,  sleep, exercise and relax. They do the same things as us, but they do them differently. On Earth, we put food on a plate. _ holds the food down and keeps it on a place. But there is almost no gravity in space, so food can float  away, and astronauts must eat carefully. Sleeping is very different in space. Some astronauts like to float in the air, but most like to be in a sleeping bag. They tie the bag to a wall so that they won't float away at night. Exercise is very important in space. On Earth,  your legs carry your body,  but in space, astronauts float, so they do not use their legs often. This is very bad for their legs, so they must exercise for thirty minutes every day. After a day's work, astronauts relax. They may listen to music, read, watch films, play games, or talk to their friends and families on the radio. Sometimes they just enjoy floating in the air. What's the main idea of the passage?
It is about the importance of gravity. It is about astronauts' everyday life in space. It is about the differences between space and Earth. It is about how to avoid   floating away while sleeping.