Why do some clothes shrink when washed in hot water and why can’t they stretch back to their normal size?
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Some clothes will shrink because they’re made from animal fiber, i.e. hair. Hair is made from the same stuff as our fingernails, keratin, which is surprising because hair is much softer. But if you put an animal hair (including a person’s hair, and even silk!) under a microscope, you’ll see a core covered by scales of keratin. So the type of material is the same–hard keratin–but by putting it together in a different way, the body has made it soft. (For a look at the scales, try this illustration from [Tricksy Knitter](https://tricksyknitter.com/blogs/learn-to-knit/why-wool-felts-and-how-to-felt-your-knitting-on-purpose) or [Knit Along Club](https://knitalongclub.com/why-does-wool-shrink/).)
These scales are larger or smaller depending on the type of animal, the health of the animal, and just individual variation. Just look at people hair–we have flat hair and poofy hair and everything in between.
When making clothes, the hairs are organized by combing them and bundling them together as strings, then the strings are woven or knit together to make fabric. Each hair still has scales, but there are so many hairs that the structure of the strings and the strength of the fabric will determine how the material behaves.
This can change, though, if the hair is exposed to heat, movement, and water all at once. Water causes the scales on hairs to open up, movement gets hairs near each other to latch onto each other’s scales, and heat both keeps the scales open longer and strips off any oils that keep the hairs from latching. Soap makes this process go faster, since it’s designed to strip away oils. When this happens, it’s called **felting**. Sometimes people do this on purpose, and sometimes it’s accidental. Light felting can sometimes be reversed, but it takes a lot of time and care to do so.
When a plant fiber shrinks, that’s a different story! A lot of shrinkage happens with cotton. This is because it’s a short fiber with a spiral shape. Take a look at the cotton plant [here](https://en.wikipedia.org/wiki/Cotton#/media/File%3ABALLS_OF_COTTON_(ANDHRA_-SOUTH_INDIA)_READY_FOR_HARVEST.jpg) and you’ll see that it’s a tiny ball of fluff. You can get fibers up to 2 inches (5cm) long from a cotton plant if you’re really lucky or if you have a very good type of plant. Because the length of these individual fibers (called “staples”) are so short, a lot of layering and spinning has to be done to make sure that the thread made from the cotton will be useful. The fibers also look like little spirals when placed under a microscope ([see this image from Wiki](https://en.wikipedia.org/wiki/Cotton#/media/File%3AC21a.jpg)).
Heat and agitation will cause cotton to shrink, just like with animal fibers above. But the cause is different! The staples of cotton try to shrink into tight spirals when exposed to heat, and because so many staples have to be used to make a string of cotton, the effect is larger. In a pure cotton fabric, this adds up to shrinking up to 20% over the life of the fabric. This shrinking is generally permanent, though like with felting, it can be reversed with time and patience.
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