Why do fingers get wrinkly in water?

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Why: When were [drinking] in the pool, it seemed like maybe it was chemicals doing it. Except that the pool was salt water!! And it happens in the bathtub, too!!

Answer: Because oil and water don't mix! You know how you're not allowed to touch paintings or sculptures* or first editions of The Velveteen Rabbit because your fingers have destructive oils? When you're submerged in water (or in the shower for way too long and use up all the hot water CHANDLER), oil soaks away from the outer layers of skin. The oils are designed to protect the epidermis and reduce water evaporation, and also to block water from entering the skin.

Once the outer oils have soaked away, your skin starts to absorb water. The outermost layer (called stratum corneum, "horny layer" lol) is made up of mostly dry dead cells, thick to protect your fingers when you dig for bugs or climb trees or build huts or scroll your mouse wheel or whatever. When the outer layer absorbs water, it expands like a sponge animal - but the underneath layers of dermis don't. What you see as pruney wrinkles are really just crests and valleys of swollen and non-swollen skin.

Science is disgusting.

*True confessions: I touched the Rosetta Stone once when I was 12. It was just sitting right there out in the open, waiting for me to put my grubby little hands all over it. I hope they've put it under glass since then.

Source: WonderQuest, SuperTight Stuff

The More You Know: While we're at it, do you know what causes goosebumps? It's a reflex (called horripilation) that causes individual muscles at the base of each hair to contract and make the hair stand up straight. It is linked to the sympathetic nervous system, which is responsible for most fight-or-flight responses - but is also constantly active, dilating pupils, activating sweat glands, constricting blood vessels, promoting ejaculation, etc. Goosebumps aren't really beneficial to humans (maybe because we don't have much fur), but they are helpful for other animals for 2 reasons:

• When it's cold, horripilation causes air to get trapped between individual hairs, causing an extra layer of insulation.
• When threatened, raised hair makes an animal seem larger and more intimidating / less fuckwithable.
  

It's always a reflex, though - not something any animal does on purpose. It's also some fun evidence of evolution.