If you are lucky enough, you can observe it early in the morning during a winter walk at the lake: the triple point of water! The water of the lake rests under a layer of ice and light fog drifts over it. You feel chilly, because the temperature is exactly 0.01 degrees centigrade. These three phases of water (liquid, solid, gaseous) must coexist in order that the temperature is a fixed point of the international temperature scale. In contrast to the ice point which depends on the air pressure and the amount of oxygen dissolved, the triple point is always exactly reproducible. Together with other fixed points (which do not, of course, include the temperature of our dummy!) it forms the temperature scale with which all other values can be determined. In addition to the realization of the temperature scale, the measuring methods as such are a special field of research at PTB, for examplethe optical detection of heat radiation or temperature-dependent noise voltages across electrical resistors.