Take a moment and look at your arm. Try to imagine the muscles in your arm and hand manipulating the bones to make them move when working to type on your computer or to move the mouse. Bones are made up of lots of living cells that help them grow and repair themselves. These living cells are constantly working to deposit hardened material in the bones to help them grow or repair breaks. Many bones are also hollow.
Hollow bones are strong and light.
There is a substance inside of the hollow bones called marrow. Marrow is the place in your body that makes new red and white blood cells. You were born with over 300 bones. As you grow, some of these bones will fuse together, resulting in only 206 bones in your adult body. The human hand has 27 bones.