This passage describes how astronauts exercise in space since there is no gravity.
Exercise is important in space. In space, there is no gravity. Everything is weightless. Astronauts float everywhere. They aren't pulled to the ground like on Earth. In space, they don't have to lift things. They don't have to push or pull things. There isn't work without gravity. This isn't healthy for astronauts. It can harm their bodies. Muscles get weak. They become smaller. This is a problem when they return to Earth. Some astronauts live in space for months. They need to keep their muscles strong. They have to exercise. This isn't easy without gravity. Scientists created special exercise machines. They are called ergometers. They are like treadmills and bicycles. Astronauts tie themselves to the machines. Cords keep them in one place. The machines put pressure on the muscles. They make the muscles do work. Astronauts exercise at least one hour every day. This keeps them fit. They return to Earth with strong muscles, just like when they left.