What is a superfluid?
A superfluid has zero viscosity. Viscosity is a measure of the fluid's internal resistance to flow, or how "thick" and difficult to pour the fluid is. Water has a relatively low viscosity and is easy to pour; treacle, on the other hand has a high viscosity, and is difficult to pour. However, as any seasoned chef will testify, treacle is much easier to pour when it is warmed, demonstrating the strong relation between viscosity and temperature.
As with other fluids, as liquid helium is cooled, the viscosity increases. However, when it is cooled below a characteristic transition temperature, the fluid enters the superfluid state. At this point,the viscosity dramatically reduces and the superfluid demonstrates some very perculiar properties.
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