When helium-4 in its liquid form is cooled to below -271.17 °C (2.71 K), it turns into a superfluid. A superfluid is a special kind of fluid that has properties that are different from normal fluids. For example, if helium-4 as a superfluid is placed in an open container, it will climb the walls of the container and spill out. This is called a "Rollin film". It is caused by a phenomenon in quantum mechanics that is known as the Clausius-Clapeyron relation. The nucleus of helium-4 has a total spin of zero (an integer), and this makes it a boson. Now, the superfluid behaviour is a form of the Bose-Einstein condensate that can only be done by bosons.