If you've heard the phrase "cotton kills," here's what that means. Cotton is absorbent, it doesn't wick moisture, it loses all insulation when wet, and water has a high specific heat. For these reasons, it's a terrible performance fiber, especially in certain conditions. Most famously, hiking in cotton is dangerous because you're probably sweating, and if you're on a somewhat chilly part of the mountain, or the clouds cover the sun, or you stopped to take a break, or whatever, and you cool down, the cotton will not insulate you. Worse, as the water cools down, it will drag the heat out of your body very fast. That's why pool water can feel cold even when it's warmer than room temperature air. All these factors mean that, even if it's just a little chilly, you can get hypothermia. Hypothermia is bad. Hypothermia is especially bad when you are stuck on a mountain and need to hike your way down. Cotton is a bad performance fiber. Wool is, generally, good. Synthetics are a mixed bag; g...