Superfluidity is a phase of matter, first observed by Pytor Kapitsa, John Allen and John Misener.
Superfluidity, like superconductivity is a quantum effect observed at low temperatures. It is typified by the complete absence of viscosity in the liquid. As well as zero viscosity it has zero entropy and infinite thermal conductivity. This means the liquid can flow without friction, it can flow through very small spaces, up capillary tubes and up and over the side of containers.

The most commonly studied superfluid is helium-4. This exhibits superfluid properties at about 2.17K. The point at which helium-4 becomes a super fluid is known as the Lambda point, because the phase diagram (Fig. 3) resembles the Greek letter Lambda. This diagram shows helium gas being cooled to form a normal liquid at the initial phase change, occurring at around 5 K. Then when the cooling continues a second phase change occurs, however it is not to form a solid, but rather to form a new kind of liquid, this is the superfluid phase.

Two interesting effects that arise from the properties of superfluidity are the fountain effect and the ability of the fluid to form a layer one atom thick covering the whole of the container it is place in.
The fountain effect, or thermomechanical effect as it is known, is particularly interesting and is a good way to demonstrate the difference between bosons and fermions. If a capillary tube is place in superfluid helium-4 and the tube is heated, in any way at all, the fluid will rise up and shoot out the top like a fountain.