Bones come in many sizes and shapes. However, the overall makeup of any bone is the same. The bone is covered with an outer layer called the periosteum. The periosteum contains blood vessels and bone forming cells. The outer layer of bone is called compact bone. Compact bone stores minerals and gives the bone hardness and strength. Inside the layer of compact bone is spongy bone. Spongy bone helps keep the bone lightweight. In the center of the bone is the marrow cavity. Marrow helps with the production of blood cells.

Bones may be broken. A broken bone is called a fracture. A broken bone may need to be 'set'. This occurs when a doctor aligns the ends of the broken bone so that the bone will grow back straight. Rehabilitation may be needed to strengthen the muscles because of inactivity while the bone healed.