superfluidity, tendency of liquid helium below a temperature of 2.19°K to flow freely, even upward, with little apparent friction. Helium becomes a liquid when it is cooled to 4.2°K. Special methods are needed to cool a substance below this temperature, which is very near absolute zero (see Kelvin temperature scale; low-temperature physics). When the temperature reaches 2.19°K, the properties of liquid helium change abruptly, so much so that ordinary helium is known as helium I and helium below this temperature is known as helium II. The transition temperature between helium I and helium II is known as the lambda point because a graph of certain properties of helium takes a sharp turn at this temperature and resembles the Greek letter lambda (Λ). Liquid helium II flows easily through capillary tubes that resist the flow of ordinary fluids (see capillarity) and a Dewar flask filled with helium II from a larger container will empty itself back into the original container because the liquid helium flows spontaneously in an invisible film over the surface of the flask. The behavior of helium II can be partially understood in terms of certain quantum effects (see quantum theory). Helium stays a liquid down to absolute zero because its zero-point energy is such that it cannot become a solid without giving up an amount of energy that is less than that allowed by the quantum theory. Similarly, quantum restrictions keep helium II from behaving like a normal fluid because the energy interactions associated with friction and viscosity in normal fluid flow involve amounts not possible for helium II.