This sudden shift into the superconducting state comes with a striking feature called the Meissner effect. The material actively shoves magnetic field lines out of itself. This effect is what lets a magnet float above a superconductor, a demonstration known as magnetic levitation.
Most traditional superconductors only work at extremely cold temperatures, cooled with liquid helium or liquid nitrogen. Despite needing such cold conditions, these materials matter enormously. They make the powerful magnets found in MRI machines, particle accelerators, and experimental fusion reactors possible.
Ordinary wires would waste huge amounts of energy as heat at the currents those machines require. A common misconception is that superconductors are simply metals with unusually low resistance. Ordinary good conductors like copper still lose a small amount of energy as heat, even at very low temperatures.
A true superconductor loses none at all below its critical temperature. This is not just a matter of degree. It is a completely different physical state.
Some newer materials, called high-temperature superconductors, work at temperatures reachable with liquid nitrogen instead of the far more expensive liquid helium. This has made superconducting technology more practical for certain uses. Scientists are still searching for materials that could superconduct at temperatures close to room conditions, which would make the technology far more practical to use.