English: Neutrons are fired from a source oven into a non-homogeneous magnetic field (S-G analyzer). Due to their 'spin' they are deflected either up or down, labelled as z+ or z-.

Only the z+ neutrons are passed to the next S-G analyzer, which this time measures the neutron's x-spin. The neutrons are deflected either right or left, called x+ and x-, and the number of neutrons with x+ and x- spin is split even. At this point it seems we have measured both the z and x spin of the neutrons.

However, this is not the case. When we pass only the x+ neutrons to the next S-G analyzer to confirm that they are all z+, we observe that their initial z-spin has been 'reset', and they are again split evenly between z+ and z-. This is despite the fact that we selected only the z+ neutrons in the first S-G analyzer.

This experiment is thought to demonstrate the uncertainty principle, that only 1 axis of a neutron's spin can be known at a time. When the second one is measured, it 'resets' the state of the first one. The reason for this is still not known for certain.