4K or -269°C is the boiling point of liquid helium. When mercury is cooled to this temperature it enters a superconductive state.
When cooled to 3K, liquid helium becomes a super fluid. It travels up and over the walls of the container forming a drop on the container bottom. Superfluidity is a very rare phenomenon that occurs at ultra cold temperatures. The key characteristic is zero viscosity.
Learn more about superconductivity.
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