I am doing some reading on Quantum Mechanics, and I do not understand this concept. Here is an excerpt from "The Universe and the Atom" by Don Lichtenberg. Sorry for the bad formatting. I don't understand why we can't answer which slit the particle when through. What else could it have gone though? This is probably a terribly literal way of viewing the event, but the whole concept is pretty fuzzy to me. I also don't understand the significance of entanglement. If two observers agree on the same axis when total spin is 0, this seems incredibly obvious. If the axises are then changed to be perpendicular of each other, the probability of prediction is 50%. Again, this seems very obvious, and insignificant. Thanks in advance to those who give wisdom.