Stiff and springy together
What Are Bones Made Of?
Bone combines a hard mineral with a stretchy protein, so it is strong without shattering.
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Step 1 of 5
Two materials, combined.
Bone is calcium mineral packed around collagen fibres. The mineral resists crushing; the collagen resists snapping.
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Step 2 of 5
Either one alone fails.
Soak bone in acid to dissolve the mineral and it becomes rubbery enough to tie in a knot. Bake out the protein and it crumbles.
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Step 3 of 5
Bones are hollow, deliberately.
A tube is nearly as strong as a solid rod for a fraction of the weight, because the outer material does most of the work. Bicycle frames use the same reasoning.
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Step 4 of 5
They are alive and constantly rebuilt.
Some cells dissolve old bone while others lay down new. Your skeleton is substantially replaced every decade.
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Step 5 of 5
They thicken where they are loaded.
Bone responds to stress by strengthening exactly where force is applied. This is why exercise builds bone and why astronauts lose it.
Bone marrow inside the hollow makes your blood cells — the space is not empty.
The short version
Bone combines hard mineral with flexible protein in a hollow, constantly rebuilt structure that thickens where loaded.
Try it yourself
Soak a chicken bone in vinegar for a week. The mineral dissolves and the bone becomes bendy.
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