Consider a cat in a box in a room. The box also contains some radioactive isotope. A geiger counter detects if the atom decays, and if it does, it sends a signal to some sort of computer, which causes a hammer to fall and smash open a bottle of poisonous gas. So, if the atom decays, the cat is dead, but if it doesn’t, the cat remains alive. We cannot tell if the cat is dead or alive until we open the box, and the dominant interpretation of quantum mechanics states that particles can exist in two states at once, and has no definite state until we measure or observe it. This means that the atom can have both decayed and not decayed at the same time. This leads to to the conclusion that the cat is both dead and alive at the same time. Now consider another room with exactly the same setup as the one previously considered. We can entangle the two cats by linking the two computers and providing them with the command that only one cat can be killed. So, if the cat in the first room is killed, the cat in the second room cannot be. Someone in room one knows that the cat is either dead or alive, but does not know which, so must say that it is both dead and alive at the same time. He therefore knows that the cat in room 2 is also both dead and alive. By opening the box, he can find a live cat, but this does not provide information about the cat in room 2 since either that cat is alive, and neither atom has decayed, or the cat is dead, and this cat cannot be killed. But now consider that the cat is dead. This means that the cat in room 2 must be alive. The person in room 1, let's call him person 1, now knows the state of both cats. However, the person in room 2, person 2, has yet to open the box, so considers both cats to be dead and alive at the same time! This cannot be true, since person 1 knows that one of them is dead, and the other is alive. The only way around this is to say that quantum mechanics does not describe reality, but only our understanding of it.