system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 How does the process of throwing up work?

I was driving about 20 minutes ago and I could feel myself about to throw up. I was less than a minute away from home and so I tried to hold it down but despite my best efforts it came up anyway and I had to open the door just seconds before spewing my guts out. Why aren't we able to hold it down? How does that bodily function work?
gpt
Your stomach is like a bag of fluid and mashed up food. It is surrounded by muscles that "squeeze" the bag. They usually just mash food around to help with digestion.

With throwing up, your body decides that it needs to get rid of your stomach contents (more on that later). It has all the muscles around your stomach squeeze hard. Picture a small Ziploc bag of water with a tube coming out the top, then you squeeze the bag hard in both hands. The contents has to shoot up the tube.

Why can't you stop it? Evolution has "decided" that over all when the situation is dire enough that you need to vomit (which is usually because you are being poisoned by something you ate), getting the poison out ASAP is more important than being able to choose where that happens. You can wash out your car or your carpet if you don't make it to the bathroom - but not if you're dead from whatever you've gone and eaten.