Final answer:
Manual labor jobs such as construction work, gardening, and using hand tools for prolonged periods can cause calluses on the hands. The skin reacts to the constant mechanical stress by thickening and forming a protective layer.
Step-by-step explanation:
Calluses can form on the hands when they are subject to constant mechanical stress, such as manual labor jobs. The basal stem cells in the skin are triggered to divide more often, resulting in increased thickness of the skin at the point of abrasion to protect the rest of the body from further damage. This is a local injury, where the skin reacts and treats the problem independently. Examples of manual labor jobs that can cause calluses on the hands include construction work, gardening, and using hand tools for prolonged periods.