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 ballet (芭蕾) dancer.“I'm an inside guy，” Mike Hopkins says, who returned from a six­month 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 (呼气) has a tendency to form an invisible (隐形的) cloud around your head.You can end up with what astronauts call a carbon­dioxide 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 you'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 of days—truly terrible days for some—astronauts' brains learn to ignore the panicky signals from the inner ear, and space sickness disappears.

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