School › Skeleton › Ages 10–11
Bones: shapes, insides and growing
The two parts of the skeleton, the five jobs of bones, five bone shapes, what is inside a long bone and how bones grow. With a paper-bone fair test.
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An adult skeleton usually has 206 bones in two groups: the axial skeleton (80 bones: skull, backbone, ribs and breastbone) and the appendicular skeleton (126 bones: arms, legs and the girdles that join them on).[1]
The skeleton has five jobs: support, movement, protection, storing minerals such as calcium and phosphorus, and making blood cells in the red marrow.[2]
Bones come in five shapes. Long bones (thigh bone, upper-arm bone, finger bones) work as levers. Short bones are in the wrist and ankle. Flat bones (skull, ribs, shoulder blades, breastbone) protect organs and anchor muscles. Irregular bones include the vertebrae. The kneecap is a sesamoid bone inside a tendon.[3]
Inside a long bone: the outside is dense, strong compact bone; inside there is also lighter spongy bone; the hollow middle (the medullary cavity) is filled with marrow.[4]
Bone is a mix of two materials: collagen fibres make it a little flexible so it is not brittle, and mineral crystals (hydroxyapatite, made from calcium compounds) make it hard.[4]
Before birth, the skeleton starts as cartilage, which bone slowly replaces. Long bones grow longer at the growth plates until early adulthood.[5]
In childhood and the teens your body builds much more bone than it removes, and bone mass usually peaks by the mid- to late 20s. Calcium, vitamin D and active play all help.[6]
Try it: Paper bones: which shape is strongest?
You need
- Several sheets of A4 paper (all the same kind)
- Sticky tape
- A square of stiff card (about 20 cm × 20 cm)
- A pile of books of similar size
- Ruler
Safety
Add books gently and keep hands, faces and feet clear of the pile in case it falls. Use books that are not too heavy, and stack them on a table, not up high.
What to do
- Question: does the shape of the same sheet of paper change how much weight it can hold? Predict which shape will hold the most books.
- Make three 'bones' from identical sheets: a flat sheet folded in half to stand up like a tent, a sheet folded into a zigzag, and a sheet rolled into a tube about 3 cm wide and taped.
- Fair test: same paper, same height, same card on top, books added one at a time, gently and in the centre.
- Stand one 'bone' upright, put the card on top and add books until it collapses. Record the number of books it held.
- Repeat each shape three times with new sheets and work out the mean. Which shape won?
What happens, and why?
The same amount of paper held far more when it was rolled into a tube. Long bones are tube-shaped too: dense, strong compact bone on the outside around a hollow middle filled with marrow, which keeps the bone lighter. Spongy bone, a lattice of thin struts, also keeps bones light while their struts line up along the directions of stress. The paper shows that shape matters; real bone is also a tough mix of collagen and mineral.[4][3]
Play: Sort the bones by shape
Which shape group does each bone belong to: long, short, flat, irregular or sesamoid?
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Quiz
Other ages: Ages 6–7 · Ages 15–16