Superfluidity is the property of a liquid that has no viscosity and therefore no resistance to flow (i.e. exhibits frictionless flow). Helium II – liquid helium at temperatures less than 2.186K, or –271°C (–456°F) – was the first known fluid. Its unusual properties were discovered independently by Peter Kapitza in 1937 and by the British physicist John Allen in 1938. They found that helium II apparently defies gravity by flowing up a slope, and if placed in a container, it climbs up the inside walls and overflows. It also appears to contradict the laws of thermodynamics by flowing from a cool region to a warmer one. The effect requires quantum mechanics for its explanation.

Superfluid helium fountain