The human body is made up of over 200 bones of different shapes and sizes. Bones are made up of living cells (called osteocytes, osteoclasts and osteoblasts) bound together by a hard, calcium-like material. This makes the bone strong and rigid. The bones are hollow and filled with a spongy material called marrow, which produces the blood cells.

The joints of the bones are covered in cartilage – a tough, flexible material, rather like gristle. As 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.

The bones have several important functions. The skeleton gives the body rigid support and the joints act as levers so that the body can move. The bones also protect organs in the body: for example, the ribcage protects the heart and lungs. They also store some of the body’s essential minerals, especially calcium.