Wool fibres aren’t completely smooth – they have microscopic scales that interlock with those on adjacent fibres when you wash them, pulling them together like a ratchet and causing them to contract. That’s why your woolly jumper shrinks in the wash.
Luckily, sheep secrete an oily substance from their skin called lanolin, which lubricates the wool and prevents the fibres from tangling when they get wet, so their fleece stays nice and full in the rain. When woolly jumpers shrink, they don’t stretch again when they dry out. So a shrunken sheep would be shrunken forever!
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