The playground at your local park probably has a conveniently spongy surface perfect for cushioning young bones upon the inevitable tumble from the jungle gym. But back in the days of monkey bars and concrete slabs, playgrounds were notorious hotbeds of bone fractures. One consolation of such a painful mishap was a cool cast signed by everyone in your class. Aside from that, a good thing about broken bones is that they start to fix themselves almost immediately. How do they do it?

Although they may appear dry and lifeless, bones are just as alive as the tissue that surrounds them. Inside the hard outer part of a bone is a spongy center filled with blood vessels and special cells that constantly tear down and rebuild the bone from the inside out. Bone cells called osteoclasts are like a demolition crew, breaking down old bone material while cells called osteoblasts are like a construction crew, building up new bone material as the old stuff is demolished.

So how do these cells help broken bones heal? When a bone breaks, a blood clot forms and bone demolition cells near the site of the break immediately begin clearing away the rubble. Meanwhile, the bone creates a sort of patch called a bone callus to replace the blood clot and hold the broken bone together. At the same time, bone construction cells near the break start to produce new bone material to repair the damage. Depending on the bone and the severity of the fracture, after several weeks its hardworking cells can repair a broken bone as good as new.