Margen
Bones look like the deadest, stillest part of your body, but they're living tissue, threaded with vessels and nerves, under construction around the clock.
Two crews work non-stop. One kind of cell, the osteoclasts, chip away and clear out old bone. The others, the osteoblasts, fill the gap with fresh material. This constant relay repairs micro-cracks, reshapes the bone to match the day's strains, and releases calcium into the blood when it's needed.
The pace is so steady that, over roughly ten years, almost your entire skeleton has been renewed. The femur you ran on as a child no longer exists: it's been replaced piece by piece.
So when someone says you're a tough nut to crack, they're only half right. Inside, you're being remade completely, and you never even notice.