Some animals have a backbone. Some do not. Animals with backbones are called vertebrates - backboned animals. Backboned animals have a hard body frame inside them - their skeleton.

Fish, amphibians, reptiles, birds and mammals have a skeleton. You are a mammal. You have a bony skeleton. Your skeleton grows with you.

Your skeleton:

• supports your body and gives it shape.
• protects the soft and squashy bits of you.
• has muscles attached to it.

Muscles are able to contract and relax. When they contract, they pull on your skeleton. You move.

Muscles work in pairs. One contracts to lift your arm. Another contracts to lower it. When you exercise, your muscles work hard.

Muscles cannot push, they can only pull. So if you want to move your arm from side to side, you need a pair of pulling muscles, working together, to do it!