I recently shocked a group of non-physicists with what I thought was a well known fact. If you are in space, in “zero-gravity”, it doesn’t feel like floating. It feels like falling.
When you are floating in water, gravity is pulling you down and the buoyancy of the water is holding you up. In particular, the water pressure pushes on your skin, and your innards are held up by the internal sinews and stringy bits inside you. But when you are falling, your organs don’t need to be held in anymore – all your bits fall together. Thus all your insides are free to move around, giving you that “stomach in your mouth” sensation.
If you are orbiting Earth, you are literally falling. It’s just that you’re moving fast enough that the force of gravity, instead of pulling you down towards Earth, pulls you around in a circle. But it feels exactly the same – your innards are orbiting with you, and your stomach is in your mouth.
What surprises me is that human beings can ever get used to that sensation. Just imagine feeling like you’ve just hit the top of a roller-coaster for months on end?