Make: the simplest electric car toy, a homopolar motor

Put round magnets on either end of a AA battery and set it down on a sheet of tinfoil and watch it spin! It's a homopolar motor, a simple electric motor that relies on the Lorentz effect to set it in motion.

Kottke explains:

How does it work? Well, it's been awhile since my last electromagnetism class, but the homopolar motor works because the combination of the flow of the electric current (from the battery) and the flow of the magnetic current produces a torque via the Lorenz force. This short video explanation should give you a good idea of the principles involved.

Cool car built from a battery and two magnets [Jason Kottke]