Our feet play an important role in getting us around. Sometimes, pressure placed on the foot becomes out of balance and extra friction falls on particular areas of the foot. When this happens, the body may respond to the pressure by producing thickenings in the surface layer of the skin. These hard patches of skin are called calluses. If the cause of pressure is not relieved, calluses become painful. Common sites of corn and callus formation, including the ball of the foot, under the big toe, tips and tops of toes and between the toes.