Imagine a gun is hooked up to a machine that measures the spin of a quantum particle every time the trigger is pulled. If the particle is measured as spinning clockwise, the gun will fire; if it’s spinning counter-clockwise, it won’t. A man points the gun at a sandbag and pulls the trigger 10 times. The gun goes off seemingly at random: “bang-click-bang-bang-bang-click-click-bang-click-click.” Then, the man points the gun at his own head and attempts to pull the trigger 10 more times. What does he hear? “Click-click-click-click-click-click-click-click-click-click.” He could keep on pulling the trigger for eternity, and the gun would never fire. How is that possible?