A need for strength makes the bones rigid, but if the skeleton consisted of one solid bone, movement would be impossible. Nature has solved this problem by dividing the skeleton into many bones and creating joints where the bones intersect.
Joints come in a variety of designs, each especially built for the limb it serves.
Some joints, like those connecting the skull's series of bones, allow no movement. The skeleton is made up of many kinds of movable joints. The bearing surface is made smooth by slippery cartilage to reduce friction. The ends of these joints are coated with cartilages which reduce friction and cushion against jolts. Ligaments then bind these bones to prevent dislocations and limit the joint's movements.