system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: On survive shows like Man vs Wild, Survivorman, etc., why can't the people just eat grass or leaves? It's not poisonous, would provide them with some nutrients, and could save their lives if the situation were real.
gpt
This is an important question. Many animals can digest foods that we people can't use at all. If we ate the grass on our lawn, it wouldn't do us much good - we would slowly starve if that were all we had to eat. Grass and other plants have plenty of the chemicals that we need to live and grow, but our digestive tracts can't digest them and so we can't use those nutrients. But an animal like a cow will eat grass or hay and little else and do just fine. What's the difference between us and the cow? The answer is in those extra "stomachs" and what lives there. 


The biggest part of the cow's stomach is called the "rumen" and it contains billions and billions of friendly germs. These germs are microorganisms that we can see only with a microscope. These germs - there are many different types in the rumen - are not harmful, in fact, they are badly needed by the cow so that it can use the nutrients in grass and other plants. The nutrients in grass are in the form of complicated big molecules that must be broken down into smaller pieces that can then be used by the cow's digestive tract and so used by the cow for growth of the body and to maintain life. The germs can do that job of breaking down those plant molecules, and that is why they are so important. So cows can live on grass and we can't - and it's all because of those extra "stomachs" and their friendly germs!