Cotton (like most plants) is made of a substance called cellulose, which contains hydrogen - an essential ingredient in water. When this material is woven into a piece of apparel - like a shirt - the hydrogen particles are attracted to each other and form a bond. This gives the shirt shape and helps it to maintain its form when it's worn, sat upon or folded - unless you get wet.
If you sweat, spend time in a humid area, spill a liquid on yourself or wash your shirt, it wrinkles. It does this because the hydrogen links in the cotton react to water, causing the fabric to bend out of shape and form wrinkles in the cloth.
So, now that you know how wrinkles are happen, how do you prevent them? Well, obviously, it's difficult to prevent ourselves from sweating when it's hot and sometimes we can't avoid humid environments - so the next best thing to do is treat the fibers with something that's waterproof to prevent the hydrogen bonds from breaking.
Today this is accomplished with non-toxic, chemical treatments, leaving your cotton clothing looking crisp and sharp all day long. This type of apparel is especially appealing to travelers, golfers - and those of us who dislike irons.
Interested in learning more about how wrinkle-resistant clothing came to be? Then take a look at our article on Ruth Benerito, Patent #3,432,252 and the Story Behind Wrinkle-Resistant Apparel.