system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do we get diarrhea, and why is all liquidy??

Just things you think about when you are awake at 4am taking a shit.
gpt
Diarrhea is the result of stool passing through the large intestine too fast and the intestine not being able to absorb all the water. Causes vary from infection, to nervousness, to too much fiber. Also, it depends on how what you're consuming reacts with your body. Alcohol causes some people to get diarrhea because it irritates the stomach lining and your body pushes it through the intestines faster, same thing with spicy foods. Capsaicin reacts poorly with a lot of stomachs and the body doesn't always break it down hence fire poops. Also, there's things like IBD and IBS. 

Source: lots of diarrhea, roommate has IBS.