system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Our saliva is the first stage of digestion and starts to break down food. Why does sucking our thumbs not break down our flesh?
gpt
First: saliva is 99% water, so anything in it will be in very low concentrations (which is why you should chew thoroughly before, so you can mix the saliva and help with very small portions).

Second: most of the enzymes present are for breaking starch and fat, both of which are not found in the outside of our thumbs.

Third: our ~~thumb~~ thumbs (and most of the body as well) are protected by the skin, made primarily of a protein called keratin (it rhymes). This is a very hardy structural protein, the same in our nails, hair and in some animal horns.