(This video has no sound) The bottom piece is cooled down so it gets superconductive. The one on the top is a magnet. The magnetic field from the magnet is causing the electrons to move in the superconductor (superconductive means electrons can flow with NO friction, so there is no resistance at all). The moving electrons in the superconductor generate a magnetic field much the same way as an electromagnet. The magnetic field generated exactly mirrors the magnet's own field, so it floats!!