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human
ELI5: why does paper harden after drying

Recently I accidently spilled water on some important notes, so I dried them on a radiator. When they were fully dry, I noticed they were much stiffer. What is the cause of that? What is the internal chemistry behind it?
gpt
Paper wrinkles for the same reason most natural fiber-based fabrics (which also happen to be mainly composed of digested cellulose) wrinkle when exposed to water.  The paper is essentially an amorphous polymer.  Wetting it heavily opens the polymer structure by allowing water to penetrate the matrix of hydrogen-bonded fibers and separate them.  This leads to a bunch of related phenomena, namely decreased wet strength (since you aren't dealing with strong intermolecular interactions anymore, just steric hindrance and the natural tensile strength of the paper fibers), volume expansion (from the expansion of the matrix) and a conformational change that comes from the impingement of water.  As the water leaves, this conformational change becomes locked in and usually results in wrinkling because that's a lower energy state than a completely flat sheet of paper.

As it turns out, you can also iron paper under low heat in much the same way you can iron cotton, linen or rayon fabric in order to remove the resulting wrinkles.  It works in much the same way, by applying heat to the amorphous polymer matrix to make it more glassy (in this case, that means essentially that it can flow) and flattening it into a new shape, which then becomes more crystalline (stiff) as it cools.