Lev Landau, at the Institute for Physical Problems in Moscow, develops from 1941 onward a complete theoretical explanation of the superfluidity of liquid helium — the phenomenon, discovered experimentally by Pyotr Kapitsa in 1937, by which helium cooled below 2.17 kelvin flows with no viscosity at all, climbing walls and escaping open containers against gravity. Landau proposes that superfluid helium should be understood not as a classical liquid but as a quantum mixture of two components: a normal, viscous component and a frictionless superfluid component, whose relative proportion varies with temperature. His theory introduces the concept of "quasiparticles" — collective excitations of the system that behave as effective particles with well-defined properties — as a conceptual tool for describing the thermal and mechanical behavior of quantum liquids, a framework that would become standard throughout later condensed-matter physics, including the theoretical understanding of superconductors and semiconductors. Landau, also the author of an influential multi-volume course in theoretical physics used to train generations of Soviet physicists, suffers a car accident in 1962 that leaves him with permanent aftereffects just weeks before being awarded the Nobel Prize, preventing him from resuming active research for the rest of his life.