Winter presents an interesting apparel challenge for those who want to play outside. Wear too many layers and you'll sweat; wear too little and you'll freeze your fanny. Lately, technology has offered up a solution: a paraffin-wax-like substance known as phase change material encapsulated in microscopic balls of heat-resistant plastic similar to that used in dishwasher-safe dinnerware. Coated onto fabrics, the phase change material melts and freezes before you do and, in the process, stores and expels heat energy. Clothing can now be engineered to respond to your body temperature-and heat up or cool down to keep you feeling just right.

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