Dave Johnson’s magnet machine has a lot of fascinating details:
This machine manipulates small spherical rare earth magnets. slicing one at a time from the end of a long chain, moving it around a bit, then dropping it back to re-connect at the tail end of the chain.
It also demonstrates a little snippet of science called eddy currents. Watch how slowly the magnet falls through the aluminum tube compared to falling through air: the falling magnet generates an electrical current in the tube, and that current in turn generates a magnetic field that opposes the movement of the magnet, slowing it down dramatically. See http://en.wikipedia.org/wiki/Eddy_current if you want more.
[Via Dug North's The Automata Blog]