system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Motion sickness

What's the physiological basis of motion sickness?
If it starts in your head, how does it affect your stomach/make you throw up?
gpt
Motion sickness occurs due to the fact that you feel motion, but you can't see it. The brain doesn't like this and thinks you might be hallucinating. The vomiting is a defense system that triggers due to the brain thinking you might have poison in your system.