The human body is made up of more than 200 bones of different shapes and sizes.
Bones are made of living cells called osteocytes, osteoclasts and osteoblasts. These cells are bound together by a hard, calcium-like material. This makes bone strong and rigid. Bones are hollow and filled with a spongy material called bone marrow, which makes blood cells.
The joints of the bones are covered in cartilage - a tough, flexible material rather 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.
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 ribs protect the heart and lungs. Bones also store some of the body’s essential minerals, especially calcium.