On the outside of the wool fibre is a protective layer of scales called cuticle cells. They overlap like tiles on a roof. The exposed edges of the cells face away from the root end so there’s more friction when you rub the fibre in one direction than the other. This helps wool expel dirt and gives it the ability to felt. Wool felts when fibres are aligned in opposite directions and they become entangled.
The scales have a waxy coating chemically bound to the surface. This stops water penetrating the fibre but allows absorption of water vapour. This makes wool water-repellent and resistant to water-based stains.
Image: University of Waikato