A hoof is the tip of a toe of an ungulate mammal, strengthened by a thick horny (keratin) covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole and the edge of the hoof wall. Hooves grow continuously, and are constantly worn down by use. "Hoof" is pronounced /ˈhuːf/ or /ˈhʊf/; the plural is either hooves (pronounced /ˈhuːvz/ or /ˈhʊvz/), or hoofs (pronounced /ˈhʊfs/).