Teaching my kids about electromagnetism, with this very easy and cool project. One AA battery, three rare earth magnets, and a length of copper wire = FUN
If you want to try this one yourself, just be aware that the battery and wires (depending on the wire gauge you use) can get quite hot. Be sure to give the battery a break every so often, and probably not a bad idea to wear safety glasses just in case.
Simple steps. Get a AA battery (other sizes will work, too, but you'll need to shape your wire appropriately), one or more round neodymium magnets with a conductive coating, and a length of bare copper wire.
Shape the copper wire so that it has a point, centered towards the top, that can balance on the top of the battery, and bring the two ends down so that they form semicircular loops at the bottom, just below the level of the bottom of the battery.
Attach the magnet(s) (I used three, so I'd have plenty of area for the wires to touch), and align the wire loop so that one side of the loop is on one side of the magnets, and the other on the other side. The wires should be able to touch the sides of the magnets, but should not touch each other.
If the wire loop doesn't start spinning on its own, you may need to give it a push. You can tweak the shape of the wire to get better balance or better contact with the magnet(s).
A quick video of some of the Gadgeteer and MIDI projects on display from myself (Devhammer) and Pete Brown at this year's Mid-Atlantic Developer Expo conference, which ran from June 27th-29th, 2012.
Among the projects shown are Devhammer's RGB LED tower demo, made possible by code written by Dave Durant, A Gadgeteer Arcade Enclosure running a Gadgeteer version of Pac Man, written by TinyCLR.com community member taylorza, my Gadgeteer Helicopter controller, and on Pete's table, his Gadgeteer project board, which was sending control info to his Sammich SID MIDI synthesizer. Also shown were a couple of Pete's microcontroller prototyping boards.