The hollow walls of the Perfect Coffee Mug (PCM) are filled with Phase Change Material (also PCM), a waxy semi-solid substance that absorbs heat to precise specifications. Depending on the amount of PCM added to the mug during manufacturing, the vessel can be "set" to an ideal target temperature. The Fraunhofer mug is set to 58º Celsius (136.4º Fahrenheit). When coffee is first poured in, the walls of the mug rapidly absorb heat from the coffee, until the liquid is at the pre-set ideal sipping temperature. Then, as needed, the walls gradually release heat back into the coffee, to keep it at the ideal drinking temperature for up to half an hour.