Hair that feels soft and smooth holds less water than hair that feels dry and coarse. That sounds backwards, but it has been tested: people handled strands with different water contents, and most called the drier one more moisturised, smoother and less damaged.

The answer is on the surface. Cuticle scales swell unevenly — the underside of each scale, and the material gluing them together, take up noticeably more water than the top. The scales lift, and the surface turns bumpy. Water is also sticky in its own right and adds friction — it is the same trick as a wet fingertip gripping a plastic bag. The brain reads that roughness as dryness, because almost everything else gets smoother when wet, not coarser.

So products promising hydration are, in practice, conditioning. They leave very fine oily deposits on the scales that smooth the unevenness and cut the friction. Undamaged hair has a layer of its own doing that job, and it wears away over time, so conditioner stands in for it.

That is also why hair feels better after a wash than after a leave-in alone. Water spreads what you apply, and it resets shape: it breaks the temporary bonds, the hair returns to its own geometry, and as it dries the bonds set it again.

Bone-dry is its own problem, though. At low humidity hair builds static and flies away, and it also turns brittle, with too many bonds, too little give, and micro-cracks accumulating from brushing. Nails make this easy to picture. Clipped dry, they crack past the cut; clipped after a shower, softened, they cut cleanly.

The right level sits somewhere in the middle, and it is lower than people assume. As for the idea that water itself wears hair out, the evidence doesn't support it.

The word hydrating on a bottle tells you nothing. Judge by whether a product leaves the hair smooth and manageable, and handle it more gently wet than dry.