What does silk's moisture regain explain if the fabric can absorb water vapor yet still feel dry against skin?

Answer: Water hides in fibers

Water hides in fibers ✓ — Correct. Moisture regain means water vapor can be held by a fiber relative to its dry weight. For silk, independent references put this near 10-11%. That is very different from a visible wet film sitting on top of the cloth, so a fabric can take up vapor before it obviously looks or feels soaked.

Surface must stay wet — A surface does not have to look wet for fibers to contain absorbed water vapor. Moisture regain is a weight-based material property, so the water is counted as fiber uptake, not as a puddle on the surface. This is why a fabric can change weight and comfort before it visibly darkens.

Silk repels all vapor — Silk is not a vapor-proof plastic sheet. Museum and textile references list substantial moisture regain for silk, which means it can interact with water vapor in the air. This helps explain why silk can feel pleasant in shifting humidity without claiming it beats every sport fabric at liquid-sweat management.