A caterpillar's sleeping bag
Where Does Silk Come From?
A silk shirt is made of thread a caterpillar spat out to wrap itself in, sometimes a kilometre of it in one piece.
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Step 1 of 5
It comes out as a liquid.
A silkworm has glands full of a thick protein liquid. It squeezes this out through a tiny hole, and the pulling and the air together set it into a solid thread as it leaves.
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Step 2 of 5
The cocoon is one single thread.
The caterpillar swings its head in a figure of eight for days, laying one continuous thread. Unwound carefully, a single cocoon can give several hundred metres without a break.
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Step 3 of 5
The shine comes from its shape.
A silk thread is a smooth triangular prism. Light hitting all those flat faces reflects at many angles at once, giving the shimmer that changes as the cloth moves.
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Step 4 of 5
It is remarkably strong.
For its weight, silk is stronger than steel and stretches without snapping. Cotton is comfortable; silk is fine, strong and light — which is why it was once worth its weight in gold.
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Step 5 of 5
The method was a secret for centuries.
China kept silk-making secret for around two thousand years, and taking silkworms out of the country was punishable by death. Whole trade routes across a continent existed because of it.
Spider silk is even stronger, but spiders eat each other if you farm them, so nobody has managed to make it in quantity.
The short version
Silk is a single long thread a caterpillar spins for its cocoon — triangular, shiny, and stronger than steel for its weight.
Try it yourself
Hold silk and cotton up to a light and tilt them. Only the silk changes brightness, because of those flat faces.
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