If you drop strong magnets onto a plate of copper, something remarkable happens: the magnets will slow down before hitting the surface, appearing to levitate slightly, like the copper is another magnetic force pushing them away. There's a fascinating explanation for this, as told in this video by .
The copper itself is not magnetic, but when its approached by magnets, the electrons on the surface of the copper being rotating. In an effort to resist the magnet's pull, the electrons briefly create their own magnetic field, which slows down the magnet's descent.
This phenomenon produces some cool visuals, but its use extends far beyond that. As demonstrated in the video, when a magnet is dropped through a coil of copper wire that isn't connected to itself, this reaction doesn't happen--the magnet drops straight through. But if the ends of the coil are connected, completing a circuit and allowing electrons to circle the whole coil, the magnet slows down in the middle of the coil. The momentum of the magnet is converted into electrical current. As we see, when the connection in the wire is replaced by an LED, the light turns on when the magnet passes through the wire.
This is how we generate most of the world's electricity, by passing magnets through coils of metal wire in a variety of ways. Slowing magnets with copper is also used often in high speed train breaking systems, as a way to slow the train without actual friction usually used in break pads.
The mechanics of this interaction are explained by , and , for those of you who want to details.