2013 © Vivian Ostrovsky
Two entangled particles are linked in such a way that the state of one of them determines the state of the other. Have you ever heard how identical twins say they can tell when the other one has been hurt? I’ve no idea how true this is for humans but with particles, that’s kind of what entanglement is like.
When behaving as waves, [particles] can simultaneously pass through several openings in a barrier and then meet again at the other side of the barrier. This "meeting" is known as interference. Strange as it may sound, interference can only occur when no one is watching.