What does "quantum mechanical" mean? We often hear statements like "an atom is a fundamentally quantum mechanical system" and so forth. But what does that mean? Yes, we can predict its behavior by solving the Schroedinger equation, but what I'm asking is, if we were given a black box that behaved just like an atom, is there any experiment you can do to it to be able to tell if whatever is in that box "is quantum mechanical" or not? So does that descriptor really mean what we tend to imply it means?