They are mostly collagen, a protein that is like a soft framework for bones, and calcium phosphate, a mineral that makes bones hard. Collagen and calcium work together to make bones strong and flexible. All together, the bones in your body form your skeleton. Your skeleton is very strong, but also very light. Most of your body's calcium is contained in your bones and teeth ! .The rest is in your blood.

Even though bones you've seen may have looked all dried up, like skeletons, yours really live and grow inside you. Bones are always growing and they're strongest in your 20s.

When you're a baby, you have more than 300 bones, but by the time you're an adult you'll only have 206 because some of your bones join together as you grow. To be able to grow, strong bones need lots of calcium and weight-bearing physical activity. Click here to find out how calcium and weight-bearing physical activity make bones strong.