Thanks to wool, even the cold snow can't penetrate to the sheep's skin.

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Wool: The Wonder Fiber

Wool has some unique properties that make it one of nature's most amazing fibers. Firstly, wool is resistant to fire. It will burn if it's held to an intense fire, but when it's removed from the flame, it will self-extinguish. The reason is that each and every wool fiber contains moisture. It's also an incredibly flexible and durable fiber; one fiber can be bent back more than 20,000 times without breaking and is said to be comparatively stronger than steel [source: American Sheep Industry Association]. To put this in perspective, a cotton fiber can only be bent 3,000 times before it breaks. Its natural elasticity makes it resistant to tearing as well. Wool fibers can be stretched as much as 50 percent of their original length when it's wet and about 30 percent when dry [source: American Sheep Industry Association].

Wool is also a pretty smart insulator. Think of it like a thermos for your body -- it can keep you warm or cool depending on your needs. It keeps you warm without overheating your body and in the Sahara Desert, Bedouins wear thin wool to keep them cool in the searing heat. The secret to both of these facts are the tiny pockets of air within each wool fiber that provide both insulation and breathability (we'll get into this little more on the next page). If that's not enough, it's also resistant to mold and mildew. It's no wonder humans domesticated the sheep in 8000 B.C.

Wool is also able to soak up as much as 30 percent of its own weight in moisture without feeling wet, which is one of the reasons it can still keep you warm even in the rain [source: American Sheep Industry Association]. The fibers have a natural crimp that helps to wick moisture away from the body. Getting this moisture off your bare skin is a key element to keeping warm in wet conditions. But there are some other more complex elements to the wool fiber that aid in warming you in the wetness, as well. We'll take a look at those on the next page.