What are human bones made of?

Bones are hard tissues that are living parts of your body, just as your brain and heart are living parts. Bones contain cells that divide and multiply, causing you to grow.
These cells are also always rebuilding the bony tissue to keep it strong.
Rebuilding happens less as people grow older. So a broken bone will often heal much more quickly in a child than in an adult.
Bones store substances called minerals, which your body uses. Calcium is a mineral. It helps to make the bones hard.
Bones have a strong covering, called periosteum.
Inside, there is a hard layer of compact bone.
A long bone, such as the thigh bone, has spongy tissue at its ends, called cancellous bone, and soft marrow in its hollow center.

Some parts of your skeleton, such as your arms and legs, have only a few long bones. Other parts, such as your hands and feet, have many small bones.

How are bones held together?

Your bones are held together by strong, flexible straps called ligaments.
The ends of the bones are covered with a smooth, rubbery substance called cartilage. This is the same kind of substance that forms the tip of your nose.
Cartilage works like a cushion so that the bones don’t grind against each other. Cartilage is covered in a liquid called synovial fluid.
This keeps the bones moving smoothly, like oil in the parts of a machine.