Such a crossover in behaviour — known as wave–particle duality — is perhaps not too hard to swallow. But quantum mechanics gets weirder. Slow down the gun so that just one electron at a time reaches the screen, and the interference pattern remains. Does each electron pass through both slits at once and interfere with itself? The obvious way to answer this question is to watch the slits as the gun fires, but as soon as you do this the interference pattern disappears.
It’s as if the electrons know when they’re being watched and decide to behave as particles again. According to Nobel laureate Richard Feynman, the phenomenon “has in it the heart of quantum mechanics. In reality, it contains the only mystery”.