The skeletal system is the framework of bones and cartilage that supports the body, protects internal organs, and works with muscles to produce movement. An adult human has 206 bones, connected at joints that allow different types of motion. Bones are not dead structures — they are living tissue containing cells, blood vessels, and nerves. Bone marrow inside certain bones produces blood cells. The skeletal system also stores minerals like calcium and phosphorus that the body needs.
Start with what students already know from the living-things course about bones and muscles. Use a model skeleton (or a detailed poster) and have students identify major bones: skull, ribs, spine (vertebrae), femur, pelvis, humerus. Explore different joint types (hinge joints at the elbow, ball-and-socket at the shoulder) by having students move those joints and describing the motion. Discuss the idea that bones are alive — they grow, they heal when broken, and they are constantly being rebuilt. X-ray images of fractures and growth plates make this concept vivid.
Without a skeleton, your body would be a shapeless mass on the floor. The skeletal system provides the rigid framework that gives your body its shape, holds you upright against gravity, and allows you to move. It is made of 206 bones in an adult, along with cartilage (a firm but flexible tissue found in your ears, nose, and at the ends of bones) and ligaments (tough bands that hold bones together at joints).
Bones do more than provide structure. The skull forms a hard, protective case around the brain. The rib cage surrounds the heart and lungs, shielding them from impact while remaining flexible enough to expand with each breath. The spine — a column of 33 vertebrae — protects the spinal cord, the main communication cable between the brain and the rest of the body. Without these protective enclosures, a simple fall could be fatal.
Inside many bones is bone marrow — a soft tissue that produces red blood cells, white blood cells, and platelets. Your body makes about 200 billion new red blood cells every day, all manufactured in the marrow. Bones also serve as a mineral bank, storing calcium and phosphorus. When the body needs calcium (for muscle contraction or nerve signaling), it withdraws some from the bones. When calcium is plentiful in the diet, bones deposit and store the surplus.
Joints are the places where bones meet, and different joint types allow different movements. The elbow is a hinge joint — it swings open and closed in one plane, like a door hinge. The shoulder is a ball-and-socket joint — the rounded end of the arm bone fits into a cup-shaped socket, allowing movement in many directions. The spine's vertebrae are connected by slightly movable joints, which is why you can bend and twist your back but not swing it like an arm. At every joint, smooth cartilage covers the bone ends to reduce friction, and ligaments hold the bones in place. The skeletal system provides the structure; the muscular system (up next) provides the force to move it.