A joint is a point in the body at which two bones connect. However, at most sites in the body, the two bones are not directly joined because while this would provide the structural support that is a fundamental function of the skeleton, it would not allow for any movement. This is facilitated by the inclusion of cartilage, a smooth connective tissue that covers the ends of the bones. Some joints – those that allow the widest range of movement, such as the knees and wrists – also contain synovial fluid, which provides extra lubrication and acts as a shock absorber. 

Bones move when muscles contract. Skeletal muscles can only pull in one direction, so always exist in pairs. For example, contracting the bicep muscle in the upper arm allows the lower arm to move towards the body, and contracting the opposing tricep muscle allows it to move away again. 

Tendons are tough bands of connective tissue that attach muscles to bones, while ligaments, which are similar in structure, hold bones together – in some cases restricting movement in order to protect a joint.