Rocket rolls a sheet of paper into a tube and stands it up. He sets a book on top. It holds!
"A flat sheet would flop," he says. "Why is the tube so strong?"
Raven turns the tube in her hands. "What do you notice about the inside?"
Nova hovers near the tube. "Bones have layers too," she says. "Look at what is outside and inside."
Most bones are made of the same layers.
| Layer | Where it is | What it is like |
|---|---|---|
| outer covering | the thin outside skin of the bone | has nerves and blood vessels |
| compact bone | just inside the covering | smooth, solid and very hard |
| spongy bone | inside the compact bone | looks like a sponge, still strong |
| bone marrow | the very middle of many bones | soft, like thick jelly |
Bone marrow has an important job. It makes most of your blood cells.
Red blood cells carry oxygen to the body.
White blood cells help the body fight germs.
Now test Rocket's idea yourself.
| Shape | Could it stand? | Books it held |
|---|---|---|
| flat sheet | ||
| rolled tube |
| Statement | True or false? |
|---|---|
| Bone marrow is hard like a rock. | ? |
| Spongy bone is inside compact bone. | ? |
| White blood cells help fight germs. | ? |
| The outer covering of a bone has nerves. | ? |
Terrific testing! Tomorrow we find out how bones guard the body's most important organs.