It can be difficult to grasp the magic behind 37.5, but Haggquist did an admirable job of explaining it the nuances of its design. “Most [fabric] is passive, it doesn’t know what your body wants,” he says. This stuff, on the other hand, contains active particles—naturally occurring minerals—that are incorporated into the fabric via a patented, top-secret method. While a lot of performance fabrics wick away sweat, 37.5 turns excess heat and moisture into a vapor. The fabric dries quickly, helping to regulate your body’s temperature. If you’re shivering, it does the opposite, holding in heat to bring you back to a normal body temperature.