The skeletal system is made up of 206 bones in the adult human body, providing structure, protecting vital organs, and working together with muscles to allow movement. Bones also produce blood cells inside their marrow and store important minerals like calcium.
Muscles are attached to bones by strong connective tissue called tendons, and they work by contracting and relaxing to create movement. Muscles almost always work in pairs, since a muscle can only pull, not push — when one muscle contracts to bend a joint, another relaxes, and vice versa to straighten it again.
Together, the skeletal and muscular systems allow the body to stand, walk, run, and perform countless precise movements, from writing a sentence to throwing a ball, all controlled by signals sent from the brain and nervous system.