Put two physicists in a room and ask them to talk about the interpretation of quantum mechanics. This is a recipe for disagreement; the mysteries of quantum theory run so deep that it’s hard to find any interpretive claims that are immune to controversy. Therefore, when thinking about quantum theory, it is a useful tactic to first focus on the macroscopic facts it predicts while ignoring the formalism and what it might suggest about the constitution of reality. I will adopt this tactic in my talk to describe the strange features of sequences of Stern-Gerlach measurements. This background will be enough to formulate the principle of “no information gain without disturbance” and the technological possibilities it implies, such as money that can’t be counterfeit and secure ways of distributing secret keys for cryptography. I’ll then move on to explain one idea about what might be going on at a deeper level – the idea of hidden variables. Finally, I’ll present Bell’s theorem, a result that reveals an uncomfortable tension between quantum mechanics and the theory of relativity.