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In the wash basin, after washing my face I saw a hair (definitely from my head) on it. It was straightening from a curl on its own! Observation: The hair was dry when it fell on the wet surface. I don't know about the behavior of water (or hair) that well to be able to explain what was happening. I hope someone can explain.

I don't even know if any physics is involved because it could simply be the hair trying to come back to its normal state after having the great fall of its life so don't mind if my question sounds silly.

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    $\begingroup$ IDK the full answer, but human hairs have structure, and that structure includes some bonds that break when the hair is wet and re-form when the hair is dry, and other bonds that are unaffected by water. $\endgroup$
    – Solomon Slow
    Commented Aug 13 at 13:18
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    $\begingroup$ Also, there is surface tension and other effects. If the water is wetting the hair, there can literally be some force pulling on the hair from the water, and that can also be mildly relevant. $\endgroup$
    – naturallyInconsistent
    Commented Aug 13 at 14:24

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The stuff hair is made of is wetted by water, which can easily soak into the structure of the hair itself. this causes the hair to swell up and lengthen as well. this tends to straighten out the hair.

Note that for years, a popular method to measure the humidity of air was to place a hair in it and measure how much longer the hair got, compared to a length measurement at zero humidity. You can fixture the hair so that when it lengthens, it rotates a needle across a dial. the dial can then be calibrated in % humidity. Cheap, simple and foolproof (but approximate).

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