New favorite spooky quantum effect: Let's say you have some bombs, with some unknown fraction duds and the rest live, and the only way to check is to try and detonate it. What fraction of bombs can be identified a priori as being live, if after every test each bomb is useless?
Obviously the classical answer is 0%, since you can never tell before it blows up. But you can set up a interferometer that uses photons to cause the bomb to "observe" a particle, which results in a 33% success rate *without actually detonating the bombs*
en.wikipedia.org/wiki/Elitzur%E…
en.wikipedia.org/wiki/Elitzur%E…
Essentially, you're exploiting quantum interference by forcing the photon along two different paths, one of which can possibly trigger with the bomb, and one of which can't. Observing the resulting detection location collapses the wave function and reveals the information. 
