No, it wasn't you who gained 10 pounds between the time you bought those new jeans and when you pulled them out of the dryer -- they shrunk. It's one of the hazards of modern-day clothing meeting a machine. Heat isn't the only factor contributing to the shrinkage -- those jeans were probably already shrunk when they came out of the washer, even if you washed them in cold water. Fabrics, both natural and man-made, or even blends, are at the mercy of stretching and shrinking from the time they come off the loom. Why they shrink and how you can restore them to their original size is a lesson in clothing manufacturing and science.
Manufacturers of mass quantities of clothing have been known to cut corners by stretching fabric before it's even patterned and cut to get a few more pieces out of a bolt. Your jeans or that new cotton T-shirt come to you in a stretched state. Once they're hit with water, hot or cold, all that tension applied to the fibers by pulling and expanding in the manufacturing process is going to return them back to their normal state: shrunk.
Washing Machine Woes
Your washing machine is the first villain causing your clothing to shrink -- especially top-loaders. The pieces slam against the sides of the machine, battering those fibers and reducing them back to their original, unstretched tension. If you pull on those jeans after they're washed, you'll see that they've already lost a size. Animal fibers are the most affected, which is why wool, cashmere and lamb's wool are labeled dry clean only. Cotton is also greatly affected by warm water. It's suggested that pure cotton pieces be washed by hand in cool water and line-dried.
The Dryer as Villain
The science of animal fibers is best understood if you envision a thread of lamb's wool -- its corkscrew construction is pulled straight once it's on the loom and made into fabric. It takes a bit of energy, such as hot water or heat, to affect the molecules in the thread. Once the two meet, the fiber goes back to its natural curly-cue state and your clothing is a size smaller. Clothing made of animal fibers should not be dried in the dryer.
The heat of the dryer doesn't shrink cotton clothing. When the items bounce around inside the dryer, slamming into the sides of the cylinder, the pounding works on the fibers. They constrict, returning to their initial length before they were stretched and made into clothes. You notice this when you tug your jeans on, squeezing into them inch by inch. Yet, after a while they fit like they did before laundering them. What actually happens is that the pressure of your body against the fibers stretches the jeans out again.
Fill a sink with lukewarm water, and add a tablespoon of baby shampoo or hair conditioner. Stir the water until it's blended and feels slippery.
Drop the piece of clothing in the soapy water and move it around until it's fully immersed with the water solution. Let it sit for at least 30 minutes.
Without rinsing out the shampoo/conditioner, roll the piece into a ball and wring as much water out of it as you can.
Lay the clothing out on one of the towels, spreading it out as smoothly as possible. Put another of the towels on top and, from the bottom, roll the towels together into a tube. Leave for 10 minutes.
Unroll the towels and place the garment on the third, dry towel. Smooth it out again, stretching as you tug. If you're unstretching jeans, pull at the bottoms to lengthen them. Use the pins to secure the stretched garment in place. Let it dry flat, or hang it up. The pins will keep it stretched and attached to the towel.