The oddness of life in space never quite goes away. Here are some examples. First consider something as simple as sleep. Its position presents its own challenges. The main question is whether you want your arms inside or outside the sleeping bag. If you leave your arms out, they float free in zero gravity, often giving a sleeping astronaut the look of a funny balled  dancer. "I'm an inside guy," Mike Hopkins says, who returned from a six-month tour on the International Space Station. "I like to be wrapped up." On the station, the ordinary becomes strange. The exercise bike for the American astronauts has no handlebars. It also has no seat. With no gravity, it's just as easy to pedal violently. You can watch a movie while you pedal by floating a microcomputer anywhere you want. But station residents have to be careful about staying in one place too long. Without gravity to help circulate air, the carbon dioxide you exhale   has a tendency to form an invisible  cloud around you head. You can end up with what astronauts call a carbon-dioxide headache. Leroy Chiao, 54, an American retired astronaut after four flights, describes what happens even before you float out of your seat,"Your inner ear thinks your're falling . Meanwhile your eyes are telling you you're standing straight. That can be annoying--that's why some people feel sick." Within a couple days --truly terrible days for some --astronauts' brains learn to ignore the panicky signals from the inner ear, and space sickness disappears. Space travel can be so delightful but at the same time invisibly dangerous. For instance, astronauts lose bone mass. That's why exercise is considered so vital that National Aeronautics and Space Administration (NASA) puts it right on the workday schedule. The focus on fitness is as much about science and the future as it is about keeping any individual astronauts return home, and, more importantly, how to maintain strength and fitness for the two and a half years or more that it would take to make a round-trip to Mars. The astronauts will suffer from a carbon-dioxide headache when   _  .
the y circle around on their bikes they use microcomputers without a stop they exercise in one place for a long time they watch a movie while pedaling