system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What actually happens to food once you’ve swallowed it and how does your body “sort” what it needs to retain and what to expel as waste?
gpt
Your intestines do all the work. The cilia lining the inside of the intestine pull all the useful bits that they can into your bloodstream, such as calories, vitamins, minerals, etc.. Everything else continues on through the intestine and eventually out of the body. Waste is whatever your body can't absorb. 

It is actually reasonably important to have a bunch of stuff your body can't absorb, like fiber, often called "roughage", in order to keep everything moving. If everything DOESN'T stay moving, you become constipated, which is the rather unpleasant experience of needing to go, but being unable to.