Klaus Sedlbauer and Herbert Sinnesbichler, of Germany’s Fraunhofer Institute for Building Physics (IBP), think coffee is perfect at 58 degrees C. So they built a coffee mug that automatically maintains that temperature, tending to cool its contents above 58 degrees, and releasing heat below 58 degrees to warm them back up. It works by use of an interstitial phase change material (PCM, Wikipedia) between the aluminum fins, which has a solid-liquid phase change temperature of 58 degrees. Above 58 degrees, the PCM melts and absorbs heat, and below 58 degrees it freezes and releases heat. Supposedly it can keep a cup of coffee at ideal temperature for 20 to 30 minutes.