Limbs don't work properly. Seasoning food can endanger crewmates. And lying your head down to sleep becomes a fight against gravity as the head drifts awkwardly above the pillow.
Astronaut Mike Massimino, now a professor of mechanical engineering at Columbia University, can attest to some of these things after blasting into space twice - once in 2002 and again in 2009.
The first weird thing he experienced particularly sticks out in his mind, Massimino told Tech Insider at a reception for the opening night of an opera called "The Astronaut's Tale" at the Brooklyn Academy of Music January 28.
"The first time I went upside down, I didn't feel like I was upside down," Massimino said. "I felt like the room had rotated around me."
This intense disorientation commonly happens to astronauts during their first few days in space. Have you ever spun around in a circle so fast and so many times that you got so dizzy and discombobulated that you couldn't tell which way was up anymore? That's kind of what it feels like for astronauts.
The body's balance and orientation gets severely messed up because the vestibular system - a collection of finely tuned systems that sense and maintain balance, movement, direction, and speed - normally needs gravity to work properly. If you're doing a headstand and can feel the weight of your body and blood rushing from your feet to your head, you can sense that you're upside down.
But when gravity flies out the window, it's hard to tell what the heck is going on.
"I felt perfectly straight, but the room was upside down," Massimino said. "That's pretty frickin' weird."