system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can the human body absorb toxins thru the skin, but you can't rehydrate yourself by sitting in water?
gpt
A large percentage of your skin is made up of a protein called keratin.

Keratin forms long chains and if you were to look at your skin under a really powerful microscope what you would see is a bunch of keratin chains crisscrossing together like a chain link fence.  But just like a chain link fence, those crisscrossing chains leave large holes between themselves that water, or anything else, could go through.

To stop water from passing through those holes, keratin absorbs a small amount of fat from your skin's oil.  That fat sits in those gaps in the keratin chain link fence, preventing water from getting through.

Because keratin absorbs fat from your skin oil like that, anything that is able to dissolve into that fat is able to use it to leapfrog past the keratin in your skin and be absorbed into your body.  There aren't a huge amount of substances like that but that's why very oily substances, like poison oak/ivy, are able to get absorbed into  your skin.