A small motor, encased in a metal cover. Attach the wires to the two leads coming from the motor case and the shaft starts to spin. Reverse the wire connections on the battery and the direction in which the shaft spins also reverses itself. Great for teaching electromagnetic concepts to junior high kids and also equally useful if you want to puzzle the bat guano out of unsuspecting elementary school science students.
LL Demos & Labs: Adaptable Electron, Electrical Conductivity Tester, Wrapping Up Electromagnets