Photo courtesy NASA
One astronaut lifts another with her finger.  See more pictures of weightlessness.
We have often seen pictures of astronauts floating around inside the space shuttle, International Space Station or Mir. While weightlessness looks like fun, it places great demands on your body. Initially, you feel nauseated, dizzy and disoriented. Your head and sinuses swell and your legs shrink. In the long term, your muscles weaken and your bones become brittle. These effects on your body could do severe damage on a long voyage, such as a trip to Mars.

In this edition of HowStuffWorks, we'll take you on an extended journey aboard the International Space Station, where we will examine what weightlessness is, what happens to your body, how these changes come about and what can be done to prevent or reverse these adverse effects.