A callus is an area of hard, thickened skin that can develop across the ball of the foot, on the heel or on the outer side of the big toe. Calluses form from repeated friction and pressure as the shoe (or ground) rubs against a bony prominence (bone spur) on the toe or foot, causing the skin to thicken from this pressure. Calluses have painful nerves and bursal sacs (fluid-filled balloons that act as shock absorbers) beneath them, causing symptoms ranging from sharp, shooting pain to dull, aching soreness. Calluses can be treated with over-the-counter callus removers, which use strong acids to peel this excess skin away after repeated application. Calluses can be prevented by switching to shoes that allow extra room for your feet and buying socks with thick toes and heels.