Liquid helium is something of an oddity. Its existence as a liquid at all is rather marginal, as shown by the ease with which it can be vaporized by tiny influxes of heat - just one watt is enough to evaporate about a litre of liquid in an hour. For temperatures below 2.17K, it behaves as though it were an interpenetrating mixture of two completely miscible fluids: a (relatively ordinary) normal fluid component, and a superfluid component which carries no entropy and whose viscosityis identically zero. It is the latter component that gives rise to liquid helium's celebrated frictionless-flow properties, enabling it, for example, to climb out of any open vessel in which it is placed.