Every so often, the growth of claws stops and restarts, just like the growth of hair. In hair, this causes the hair to fall out and it is replaced by a new one. In claws, this results in an old layer, and it breaks off. This takes several months for human thumbnails. Cats are often seen working old layers off on wood or on boards made for the purpose. Ungulates' hooves trim themselves when they walk on the ground. Domesticated equids (horses, donkeys and mules) usually need regular trimming by a farrier.