The Meissner effect causes a superconductor to expel magnetic fields from its interior.
Walther Meissner and Robert Ochsenfeld discovered the effect in 1933. When a material is cooled below its superconducting transition temperature in an applied magnetic field, it actively excludes magnetic flux from its bulk, apart from a thin penetration region near the surface.
This behavior distinguishes superconductivity from merely having zero electrical resistance. A hypothetical perfect conductor that was cooled in a pre-existing field could preserve that field, whereas a superconductor reaches an equilibrium state that expels it. The effect results from persistent surface currents and is explained microscopically by the superconducting condensate.
Magnetic levitation demonstrations often show the Meissner effect, although flux pinning can stabilize a superconductor above a magnet or along a track. Real superconductors have limits: sufficiently strong magnetic fields, currents, or temperatures can destroy superconductivity.