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'Helium-4' is a non-radioactive and light isotope of helium. It is by far the most abundant of the two naturally-occurring isotopes of helium, making up about 99.99986% of the helium on earth. Its nucleus is a boson and an alpha particle, having two protons and two neutrons. Since the alpha decay is a common decay mode for many radioactive isotopes. In fact, alpha decay of heavy elements is the source of most naturally occurring helium-4 on earth. Helium-4 is also a byproduct of nuclear fusion in stars.
When helium-4 is cooled to below 2.17 kelvins (–271 °C), it becomes a superfluid, with properties that are very unlike those of an ordinary liquid. For example, if helium-4 is kept in an open vessel, a thin film will climb up the sides of the vessel and overflow. Another name for this property of Helium is Rollin film. This strange behaviour is a result of the Clausius-Clapeyron relation and cannot be explained by the current model of classical mechanics nor by nuclear or electrical models; it is only understood as a quantum mechanical phenomenon.