Coffee Joulies are polished stainless steel shells full of a very special phase change material (an ingredient in food) that melts at 140°F. When you put them in your coffee this PCM begins melting, absorbing a lot of heat in the process and cooling your coffee down much faster than normal.
Where does all that heat go? It’s stored right inside your Coffee Joulies™. When your coffee reaches 140°F (the perfect drinking temperature) the molten PCM begins solidifying again, releasing all that energy back into your coffee to keep it at a comfortable and delicious drinking temperature. The more heat you feed your Joulies, the longer they’ll keep your coffee warm.