Okay, here goes... Our teacher set a question in the last test which asked us to show that if a system initially be in a stationary state, it will remain in a stationary state even if the system evolves according to the time dependent Schrodinger equation. What I did was show that the expectation value of the operator will not change using ∂<O>/∂t = 0 But now that I think about it, I find it really stupid!! Why shouldn't the expectation value change with time? It's a quantum system after all... it's supposed to be unpredictable every instant! If I know what it is now, I shouldn't know what the system will become 2 mins later,am I right? Anyway, I tacitly assumed that ∂ψ/∂t = 0 and ended up with that result... What the teacher wanted was <O(t)> = <O(t0)> Please help me get out of my own mess!! Let me know if I need to clarify anything. Thanks in advance.