My favorite I've seen personally was a science toy which supposedly lit a light by generating a current with a magnet sliding thru a coil of wire. Taking it ap

My favorite I've seen personally was a science toy which supposedly lit a light by generating a current with a magnet sliding thru a coil of wire. Taking it apart there was a hidden watch battery lighting the light! Not sure if it was cheaper to fake it or if they couldn't get the real version to work reliably. It was funny though, a faked science demo product.