The skeleton, muscles and movement

Three jobs of the skeleton

Bones do three things: they support the body so it does not collapse, they protect soft organs (the skull guards the brain, the ribs guard the heart and lungs), and together with muscles they allow movement. Naming which job a particular bone does is a common question.

Joints

Bones cannot bend, so movement happens at joints where two bones meet. The elbow and knee are hinge joints and swing one way, like a door; the shoulder and hip are ball-and-socket joints and turn in many directions.

Muscles work in pairs

A muscle can only pull, never push. So muscles come in pairs: the biceps contracts to bend the arm, and the triceps contracts to straighten it again. When one contracts, its partner relaxes.