layering

On Saturday, my friend Ryan reviewed my layering system with me for cold weather riding, I'm ok down to 20 degrees with less than 15mph winds. Beyond that I'm in trouble. After his review it was time for some shopping. I thought the base layer I was wearing was all but turns out 60% cotton. I think I just assumed. Having grown up with the expression “cotton kills” it never occurred to me that someone would make a base layer out of cotton. As I shopped though, I saw that many people do. Is there something I don't know?