Show Me Some Science! Magnetic Braking
A magnet is dropped on a cold block of copper and all sorts of awesome science ensues.
Copper is not a magnetic metal. However, when you move a magnet near a copper, you create little loops of electricity in the copper called eddy currents.
Since energy can not be created or destroyed, the electric energy in the copper has to come from somewhere. In fact, it comes from the kinetic energy of the falling magnet, which has to slow down as it looses energy to electricity. When cooled down with liquid nitrogen, the copper becomes a better electrical conductor, and more electricity is able to create eddy currents, so the effect is stronger.
This is the basis of eddy current brakes, which are sometimes used to stop trains and roller coasters.