Hi, In QM, when we make a measurement, our wavefunction collapses into one of the eigenstates of the operator. This process is supposed to be random for any single measurement but obeys some statistics if we make a large number of measurements. Could someone explain how we know that it is indeed random for each measurement? I guess this should have been the first question I asked when I started QM but somehow, I never did. At first, this sounded similar to saying that there is a hidden variable which determines which eigenstate it collapses into, so I thought that Bell's inequalities deal with it. But everything I can find about Bell's inequalities is talking about entanglement and non-locality and other things that I don't really know much about. Thank you.