  The mainstream textbooks of quantum mechanics explains the quantum state
collapses into an eigenstate in the measurement, while other explanations such
as hidden variables and multi-universe deny the collapsing. Here we propose an
ideal thinking experiment on measuring the spin of an electron with 3 steps. It
is simple and straightforward, in short, to measure a spin-up electron in
x-axis, and then in z-axis. Whether there is a collapsing predicts different
results of the experiment. The future realistic experiment will show the
quantum state collapses or not in the measurement.
