In the early part of the twentieth century, liquid helium was found to have zero viscosity at temperatures below about 2.2 K. The condition of matter displaying this property was named the superfluid state. This property was later explained as a consequence of the fact that helium-4 (⁴He) atoms are Bose particles which, at sufficiently low temperatures and sufficiently high density, can display the quantum statistical phenomenon called Bose-Einstein condensation. This occurs when a fraction of the atoms occupy the same zero-kinetic-energy quantum state.