Bones are linked together by joints. Most joints allow different parts of the skeleton to move. The human skeleton has joints called synovial joints.
If two bones just moved against each other, they would eventually wear away. This can happen in people who have a condition called arthritis. To stop this happening, the ends of the bones in a joint are covered with a tough, smooth substance called cartilage. This is kept slippery by a liquid called synovial fluid. Tough ligaments join the two bones in the joint and stop the joint falling apart.
Different types of synovial joint allow different types of movement. The table describes two types of joint:
|Type of joint||Examples||Movement allowed|
|Hinge joint||Knee, elbow||The same as opening and closing a door, with no rotation (turning)|
|Ball and socket||Hip, shoulder||Back and forth in all directions, and rotation|
The bones cannot move on their own - they need muscles for this to happen.