Helium boils at a substantially lower temperature, 4.2 K (−452°F or −269°C), than any other substance; and below 2.172 K (−455.76°F) the liquid exhibits the extraordinary properties of superfluidity, notably the ability to flow through narrow channels with complete absence of friction. In addition to the common isotope of atomic weight 4, helium has a rare isotope of atomic weight 3 with a normal boiling point of 3.2 K (−454°F) and a superfluid transition at a very much lower temperature near 0.001 K. Both forms of helium remain in a liquid state at absolute zero. All of these characteristics are due to the weakness of the attractive force between two helium atoms and to the small atomic mass, which according to the laws of quantum mechanics makes the atoms difficult to localize.