Author(s): John Beamish 
At temperatures below 2.176 K, helium-4 enters a superfluid state and flows without friction. This 'perpetual motion' makes superfluidity -- perhaps even more than its electronic counterpart, superconductivity -- the most dramatic manifestation of quantum mechanics on a macroscopic scale. Despite its appeal, and despite many searches for superfluidity in other systems, it remains an uncommon phenomenon. From 1938, when superfluidity was discovered [1, 2], helium-4 was the only known example until 1972 when the phenomenon was seen , at much lower temperatures, in helium-3. That temperature difference between the two isotopes' behaviour reflects the intimate connection of superfluidity to Bose-Einstein condensation -- the transition that occurs when 'bosons' collect in a single quantum mechanical state. Atoms of helium-4 are bosons, but helium-3 atoms are 'fermions' and must pair up before they can condense into a single state.
In 1995, advances in laser-cooling and magnetic-trapping techniques led to the achievement  of Bose-Einstein condensation in rubidium vapour, adding to the list of superfluid systems. That list now includes other gases, such as spin-polarized hydrogen gas , and, most recently, molecular gases of paired fermions [6, 7]. On page 225 of this issue, Kim and Chan  claim the first observation of superfluid behaviour in a solid. A sample of solid helium-4, confined in the nano-scale pores of Vycor glass and rotated in a torsional oscillator, underwent a transition below about 175 mK that indicated the onset of 'supersolid' behaviour (Fig. 1). If it can be confirmed that superflow is occurring in the solid helium, this is a remarkable result indeed.
Despite their rarity, superfluids are fundamental to, for example, statistical mechanics and fluid dynamics, and they are a valuable test bed in fields as diverse as turbulence and cosmology. So it is not surprising that superfluidity has been sought in new systems. Solids, with their atoms localized on a periodic lattice, are certainly the most unexpected phase of matter...