Our bones are constantly changing through a process called bone remodeling. There are three primary cells responsible for bone growth throughout your life. Cells called osteoclasts are constantly breaking down old bone so that other cells, called osteoblasts, can replace it with new bone tissue. Osteoblasts add minerals to make bones hard. New cartilage is formed from another type of cell called a chondroblast. Thanks to osteoclasts, osteoblasts, and chondroblasts, your bones begin to heal on their own even before you get your injury is treated. As soon as you break a bone, your body responds immediately to repair the injury. Within two hours, a blood clot forms around the break. Immune system cells inside the clot, called phagocytes, begin cleaning bone fragments and removing unwanted bacteria or material around the break. Next, chondroblasts creates a soft callus, made mostly of collagen, around the fracture. This stage can last anywhere from 4 days to 3 weeks. Two weeks after the injury, the bone remodeling begins and lasts another four to ten weeks more.