A blister is a fluid filled sack that forms under the skin and is caused by friction. Calluses and corns are generally also caused by friction, but do not contain fluid. A callus results when friction causes a build up of dead skin, often on the heel or bottom of the foot, whereas corns, small and kernel shaped, often form on the top or between toes. Some corns may need to be professionally removed, but for calluses, a callus tool such as Tweezerman’s Sole Mates Foot File. To keep your feet healthy, keep in mind that shoes that are too tight or too loose can rub against and irritate the skin.