Activities that are known for causing calluses include (but are not limited to) construction work, many sports, wood carving, playing musical instruments, use of a chef's knife, rock climbing, hiking, martial arts, weight training, rowing, BMXing, dancing (especially ballet), chopping wood, monkey bars and wearing high heels. Tenpin bowlers will often develop calluses on their thumbs and occasionally their middle fingers from frequent bowling. Calluses have also been known to develop on the forehead from the frequent prostrations required in Muslim prayer; known as a prayer bump or "zebiba", such calluses are considered marks of piety in some Muslim countries, and people have been known to take special steps, such as praying on straw mats, to encourage the callus to develop.