system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does getting nutrients from other organisms' feces work and how come humans can't do the same thing?

I mean I know it'd be pretty gross if humans ate poop on a regular basis but if it was for survival purposes then I would be somewhat okay with it.
gpt
Digestive systems are never perfect, organic life is too complex. Often one animal's digestive system will be a bit crap (heh) at getting a particular range of nutrients out of their preferred food source. In these cases, another animal that lives in the same ecosystem may well smell their feces, realise it smells like food (many species are good at smelling nutrients that are important to them) and eat it. They may be better than the first animal at getting the leftover nutrients out of the pre-digested food.

Humans usually don't do this because our digestive systems are very good at getting a wide range of nutrients out of a wide range of food, and because there's a high risk of getting sick if we eat shit.

However, we're not very good at breaking down the seed coat of some plant species, which is why whole corn kernels may turn up in your poo unbroken if you don't chew your corn up properly when eating it. For this reason, some non-agricultural (hunter-gatherer) peoples in food-scarce situations may practice "second harvesting" - going through their own or others' poo to find unbroken, undigested grains and seeds to eat.

TL;DR: you can if you really want, but get ready to get gastro.