A callus is a thickened area of skin that forms in response to repeated friction, pressure, or irritation. It typically develops on the hands or feet, where the skin is often subjected to stress. Calluses serve as a protective barrier, helping to prevent blisters and other injuries. The formation of a callus is a natural process, as the body adapts to protect itself. Common causes include wearing ill-fitting shoes, using hand tools, or engaging in activities like running or playing musical instruments. While calluses are generally harmless, they can sometimes become painful or lead to other skin issues if not managed properly.