Most of us are familiar with calluses. These extra-tough patches of skin are generally quite useful–we might even say handy–because they act like a kind of natural armor, protecting areas of skin that get an unusual amount of wear and tear. Have you ever wondered exactly what a callus is, or how it forms on your skin? Callus formation is triggered by pressure or abrasion. The heel of your foot as it rubs inside your shoe, or the palm of your hand if you’re doing a lot of manual labor are good examples of this. While it might feel like a callus is something extra, added to your skin, it’s actually just a build-up of what’s already there. Here’s How It Works Your skin has a number of layers of different types of cells. The outermost is a layer of hardened, dead cells. This top layer is usually about twenty-five cells thick, and it constantly replaces itself as the outermost cells flake off to be replaced by new hardened, dead cells underneath.