The bones inside your body make a hard frame called the skeleton. The bones hold up your body and give it its shape. They also protect you. The bones of your skull protect your brain. Your ribs form a cage that protects your heart and lungs. A grown-up's skeleton has 206 bones, which is a very big number. Let's find out what all of those bones are made of.

You may think bones are dry sticks, but bones are living tissue. Bone is a framework of the protein collagen made hard by the mineral calcium phosphate. The outside of a bone is made of hard, dense bone, and soft bone marrow inside many bones makes most of your blood cells. Let's look closer at how bones are built.

Bones come in four main shapes, which are long, short, flat and irregular. Long bones grow longer at growth plates near their ends, not from the middle. At birth, the skull bones are not firmly joined, which leaves soft spots on the head. As a child grows, the skull bones that cover the brain fuse together at joints called sutures.

Most muscles are attached to bones, and the bones act as levers that let body parts move, and tendons connect muscles to bones. A joint is a place where two or more bones meet. Cartilage covers the ends of bones in a joint and helps them move without rubbing. The fingers of each hand have 14 bones. Each finger has three, and the thumb has two.

What if a bone breaks? Let's see what doctors do. Doctors use X-rays to see whether a bone is broken, and a cast or splint holds a broken bone still while it heals. Children's broken bones can heal well and heal quickly, and bones also get stronger when you use them by being active.

Not every skeleton is the same. The part of your ear you can see is made of cartilage, a tough bendy tissue, not bone. 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.

Words to know

• collagen: bone protein
• sutures: skull joints
• cartilage: bendy tissue
• marrow: soft inner part

The short version

Bones are living tissue. Bone is a framework of the protein collagen made hard by the mineral calcium phosphate. A grown-up's skeleton has 206 bones, which hold up the body, give it shape and protect the brain, heart and lungs.

Quick facts

• A grown-up's skeleton has 206 bones.
• Bone is a framework of the protein collagen made hard by the mineral calcium phosphate.
• Long bones grow longer at growth plates near their ends, not from the middle.
• Bones come in four main shapes: long, short, flat and irregular.
• Soft bone marrow inside many bones makes most of your blood cells.