When skin is cut, torn, or burned, the body responds by sending its cleanup crew: the inflammatory cells that eat invading bacteria and release, both internally and externally, "cytokines," which kill the invaders and, often, recruit additional defensive cells. Some of these cells are fibroblasts, simple but strong cells that secrete tough products which plug and give strength to the site of injury. It is interesting that scarring usually doesn't occur if a fetus is injured any time in the first six months of pregnancy (when the body has little inflammatory response to injury), though scars may form if the injury occurs in the final three months.