The 1-K pot is a small vessel in a cryogenic system that is filled with liquid helium. Usually it is a few cubic centimeters in size with a pickup-tube extending into the primary liquid helium bath of the dewar. When a vessel containing liquid helium is connected to a vacuum pump, the vapor pressure above the liquid surface drops, thereby allowing the more energetic helium molecules to evaporate out of the liquid. As the particles evaporate and are pumped away from the liquid, they carry heat energy with them, so the remaining fluid tends to cool. This technique is known as evaporative cooling.