The human skeleton is made up of more than 200 bones of different shapes and sizes.

Bones are made of collagen (a type of protein) and minerals such as calcium and phosphate. These make bones strong and rigid.

Bones do lots of important things:

• They support our bodies and the joints at the end of bones act as levers so we can move around.
• They protect various parts of our bodies from injury, for example the ribs protect the heart and lungs.
• They store important minerals that are used by our bodies, such as calcium.
• Some bones are filled with a soft, spongy material called bone marrow, which makes blood cells.

Our bones are constantly being broken down and replaced with new bone. This helps maintain their strength and shape. There are two main types of cell that break down and replace bone:

• osteoclasts break down and remove old bone
• osteoblasts make new bone.

The joints at the end of bones are covered in cartilage. This is a tough, flexible material, like gristle. Because cartilage is more elastic than bone, it allows the bones to move freely at the joints. It also cushions the bones at the joints, to stop them rubbing against each other.