The bones inside your body make a hard frame called the skeleton, and a grown-up's skeleton has 206 bones. The skeletal system, with its bones, cartilage, ligaments and tendons, makes up about 20 percent of body weight. A skull has 8 bones in the part that holds the brain and 14 bones in the face. The femur, or thighbone, is the longest and strongest bone in the human body. Some bones are guards: the bones of your skull protect your brain, and your ribs form a cage that protects your heart and lungs.

Let's look at one long bone from end to end. A long bone has a hollow shaft (diaphysis), flared ends called metaphyses below the growth plates, and rounded ends called epiphyses above them. Long bones grow longer at growth plates near their ends, not from the middle. Cartilage in the growth plate keeps growing until it stops in the early twenties, when the plate turns to bone and the bone can no longer grow longer. Bone mass, the amount of bone tissue in the skeleton, peaks by the mid- to late 20s.

You may think bones are dry, dead sticks, but bones are living tissue. The outside of a bone is made of hard, dense bone, and inside that hard layer, many bones have a lighter, sponge-like tissue. Soft bone marrow inside many bones makes most of your blood cells, while yellow marrow is found in the large cavities of large bones and is mostly fat cells. Old bone is constantly broken down and replaced by new bone, a process called remodeling, and bone-building cells (osteoblasts) and bone-removing cells (osteoclasts) balance each other to keep bone healthy. Bones get stronger when you use them by being active.

A baby's skull bones are not firmly joined at birth, which leaves soft spots on the head. As a child grows, the skull bones that cover the brain fuse together at joints called sutures. Bones come in numbers and shapes, and each hand has 27 bones: eight in the wrist, five in the palm and 14 in the fingers. A joint is a place where two or more bones meet, and ball-and-socket joints, like the hip and shoulder, can move forward, backward, sideways and in circles. Cartilage covers the ends of bones in a joint and helps them move without rubbing. Most muscles are attached to bones, and the bones act as levers that let body parts move, and tendons connect muscles to bones.

Not everything stiff in your body is bone. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone, while the stapes, one of three tiny bones in the middle ear, is the smallest bone in the body. A shark has a skeleton made of cartilage instead of bone. A giraffe has seven bones in its neck, the same number as a person. A bird's bones are hollow and light, which helps it fly. Skeletons come in more than one kind, and each kind has its own parts.

Doctors see and fix bones, and they use X-rays to see whether a bone is broken. Wilhelm Röntgen found X-rays by accident in 1895 while experimenting with vacuum tubes, and his X-ray picture of his wife's hand showed the bones of her hand and her ring. Bones show up white on an X-ray because they absorb the most radiation. A cast or splint holds a broken bone still while it heals. Children's bones are softer and more likely to bend than to break all the way through, and children's broken bones can heal well and heal quickly. Broken bones heal in three stages: inflammatory, reparative and remodeling.

Words to know

• diaphysis: middle shaft
• epiphyses: rounded bone ends
• osteoblasts: bone-building cells
• marrow: soft inner part
• remodeling: old bone replaced