The living bone is continuously repairing itself.
This repair process is called "Bone Remodeling". Whenever there is a tiny crack inside the bones, for example, after landing from a high jump, the bone cells will dissolve the area around the crack and fill it in with new bone. At any one time this is happening in millions of places throughout the skeleton.
The first movie shows what happens in one place. You'll see a little crack, then cells will turn into osteoclasts and dissolve (resorb) the bone, then cells from the marrow space will turn into osteoblasts and build new bone.
Click inside the frame to see the movie. It is 432 kb and may take a few minutes to load. Click on the movie to see it again.
The second movie shows this process on a larger section of bone where at least ten places are getting remodelled.
This movie is 568 kb.