Mathematical hats for very low temperatures

Klein Bottle Hats (http://kleinbottle.com/klein_bottle_hats.htm) : not only nifty mathematical objects themselves, but the sales page includes some intriguing sentences -

Warm in the winter? Yes, indeed! They may be single-sided but they're double-walled, so your head enjoys twice the insulation of a standard ski-hat. There's two layers of wool between your head and that cruel winter wind. Works down to minus 40 degrees (F or C, your choice) !