Incredible shrinking clothes.

A Textile scientist explains what happens when clothes get smaller.

You know the problem. You buy a pair of jeans that fit just right--then you wash them.

Why do clothes shrink? To find out, I called textile scientist Vasudha Ravichandran.

Taking a break from her current research project--how to get grease out of mechanics' overalls--Ravichandran opened my eyes to the wide world of textiles.

For one thing, textile scientists deal with more than clothes. Some work in high-tech fields, designing airplane wing materials. Others create optical fibers, hair-thin glass filaments than can transmit phone calls or TV programs at the speed of light. Still others are biology experts, studying how enzymes help target body oils on clothes.

With this sort of knowledge under her belt, I figured Ravichandran would surely know why jeans shrink. Here's what she told me.

Why do jeans shrink?

Believe it or not, shrinkage is a "bonding" experience. The cotton fibers of your jeans are made of lots of small molecules, linked together to form huge chains of molecules called polymers. Weak links called hydrogen bonds connect the polymer chains end-to-end. When the bonds break, the polymers crinkle up. Result: shrinkage.

Could you start at the begining?

Sure. Let's go back to when your jeans were made. When cotton fibers are spun into threads, they--and the polymers they're made of--are first pulled and twisted. That puts stress on the hydrogen bonds.

The bonds are stressed even more before weaving, when the threads are stretched on a loom. So much stress breaks the bonds. But new ones form to hold the polymers in the "stressed out" state.

Of course, the polymers "want" to return to their natural relaxed state.

To do that, they need bond-breaking energy. You help by throwing your jeans in the wash.

How? Chemicals, such as water in your washing machine, or heat in the washer or dryer privide the energy needed to break the stress-producing hydrogen bonds. When the bonds break, the polymers crinkle up and relax. "That's when shrinkage happens," says Ravichandran.

What about the sweater I washed? It fits my cat now.

Wool shrinks because of the structure of sheep's hair. Like human hair, wool fibers have scales that are stacked like roof shingles. (Try this: Pluck a long hair. Rub your fingers along it from the root to the tip; then in reverse. Which direction is smoother? Look at the hair under the microscope to determine why.)

When wool fibers hit hot water or high temperatures in the dryer, the scales stick out like thorns. They snag one another, clumping the fibers. Result: one sweater, size XS.

Can't they do anything to prevent shrinkage?

Read some clothing labels. Cotton manufacturers, for instance, often add chemicals called shrink-resistant or durablepress finishes. The finishes form cross-links between the molecules of parallel polymer chains. The cross-links resemble ladder rungs connecting one polymer to another. They allow the polymers to withstand the stressful stretching of manufacture. The low-stressed polymers won't have the same "need" to crinkle up and relax during washing.

What can I do?

Try these tips:

* For cotton T-shirts buy one size larger. They may shrink up to 22 percent in the wash.

* Check labels for natural-synthetic blends, such as cotton-polyester. Polyester fibers don't absorb as much water (i.e., bond-breaking energy) as cotton fibers do, so shrinkage will be limited.

* Try "presashed" clothing. This fabric has been washed by the manufacturer several times to release the tension before you buy it.