Why is wool so warm?
For grown-ups: the science
Wool is warm because its curly hairs trap still air that helps keep your body heat close.
Wool fibers have natural crimp, creating a lofty structure with many air pockets. Air has low thermal conductivity, so trapped, relatively still air slows heat transfer from the body. Wool can absorb water vapor and some moisture while retaining much of its insulating structure, though it insulates less effectively when very wet.
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Transcript
A wooly sweater feels soft and cozy. Why does wool keep us so warm?
Wool is made of many curly animal hairs. The curls make lots of tiny spaces between the hairs.
Those tiny spaces hold air close to your body. Still air does not let your body heat escape quickly.
Wool can stay warm even when it gets a little damp. Its hairs hold some water while keeping many air spaces.
So wool is warm because its curly hairs trap still air near you.