“When the temperature of liquid helium falls below 4K (-269°C), it is at first simply a colourless liquid, albeit extremely cold, but when the temperature drops below 2K something rather strange happens to it. It becomes a liquid with bizarre properties and is known as helium-II. Not only does it expand on cooling, but it is a million times more heat conducting than normal liquid helium and its viscosity is zero, in other words, nothing can slow its flow. It will escape from any container that is not totally sealed; it does this by flowing up the sides and out of the top, apparently defying even gravity.”
– Nature’s Building Blocks [pg. 178].