Wrap a light bulb inside of a handkerchief, and allow a spectator to wave it over several piles of cards. When the bulb comes close to one pile, it lights up! Sure enough, the top card of that pile is the spectator’s selected card.
The light stays on until you are ready for it to go off. To turn the light off, you cut an “invisible” wire with a pair of scissors. At the instant you snip the invisible wire, the bulb goes out.