The bones in a joint are shaped to fit together. This can be seen well in the hip joint. The hip joints carry the weight of the head and body onto the thigh bone at the top of the leg. The hip joint is made between the top of the thigh bone and part of the pelvis. The top of the thigh bone is in the shape of a ball. It fits into a cup-shaped hollow in the pelvis. This hollow is called the socket and the two bones together make a ball and socket joint. This kind of joint also allows a great deal of movement as well as great strength. Another large ball and socket joint is found at the shoulder. It is made by three bones - the upper arm bone, the shoulder blade and the collar bone. If you want to see how much movement this joint give you just wave your arms about.

Bones will not stay in their joints without support. They are held in place by ligaments. These are sheets of tough material which do not stretch. Despite their strength they can be torn if the joint is put under a great deal of stress. Torn ligaments are a common injury in sports such as soccer.

When a bone moves in a joint it rubs on the other bones in the joint. This could wear down all the bones and create painful friction. The parts of the bone that meet in a joint are protected from wear and friction by cartilage. In the spine the cartilage takes the form of discs. Each vertebra is separated from the next by a disc of cartilage. In addition to protecting the bones the discs also act as shock absorbers as you walk, run and jump. They prevent the body jerking harshly whenever the feet touch the ground. During the course of a the day the discs get squashed a little flatter but at night when you are laid asleep the discs take in some water from the blood and swell up a little again. This means that you are at your tallest when you get up and become fractionally shorter during the day as your discs become slightly flatter.

In joints like the ball and socket joint the parts of the bones in the joint are coated with cartilage. The cartilage has a tough surface to prevent wear and a slippy surface to prevent friction. The bones are also further protected by a liquid in the joint. It acts like an oil and lets the bones move smoothly over each other.