What water changes inside a hair strand
A hair shaft is built largely from keratin, a protein arranged in long, bundled structures. Those structures are held in place by several kinds of chemical links. Water can loosen some of the weaker, temporary links—especially hydrogen bonds—without dismantling the stronger framework that gives hair its shape. As water moves into the strand, the fibers also swell. Together, these changes let wet hair bend and lengthen more readily.
That extra give can be useful: a strand can stretch around a comb or a tangle instead of resisting immediately. But it also means the strand can be pulled farther out of shape before the force is released.
Why brushing can cause more damage when hair is wet
Brushing applies tension as the teeth pass through knots. A dry strand is generally less stretchy, so it may resist that pull; a wet strand can elongate, and repeated pulling can strain its protein structure. If the load is too great, the strand may break. Rough handling can also damage the hair’s outer cuticle, the overlapping protective layer that helps shield the inner shaft.
The practical distinction is between flexibility and durability: being able to stretch farther does not mean a strand can withstand more force. Detangling gently, working from the ends upward, reduces the tug each knot passes along the hair. That matters most when hair is wet—not because water makes it instantly fragile, but because a flexible strand is easier to overpull.
