The Stern-Gerlach experiment is often cited as evidence of quantum superposition and there are some very simple explanations like this one https://www.youtube.com/watch?v=hkmoZ8e5Qn0 in which spin on one axis is compared to colour and spin on a perpendicular axis is compared to hardness.
The Stern Gerlach experiment relies on the inherent angular momentum of a silver atom giving it the properties of a tiny bar magnet. When a beam of particles are fired through a large external magnet field, half of them are deflected up and half are deflected down, with respect to the orientation of the external field. This does not surprise me at all. I am reminded of a high school experiment in which iron filings sprinkled onto a piece of paper which is placed over a bar magnet to demonstrate the lines of force. I imagine something similar happening when a beam of atoms are fired through the Stern Gerlach apparatus; the particles align themselves with an external (UP/DOWN) magnetic field as they pass through it. $50$% go up and $50$% go down. When only the UP aligned particles exiting the apparatus are directed through a second apparatus, at right angles to the first apparatus (LEFT/RIGHT), $50$% go left and $50$% go right (that is up or down with respect to the alignment of the second apparatus). This is no surprise to me at all because I imagine them simply realigning themselves to the magnetic field.
When ALL of the particles exiting the second apparatus (which are now aligned either left or right) are directed through a third apparatus, with the same orientation as the first (UP/DOWN), they ALL come out aligned UP. Admittedly this seems odd, but perhaps they have retained enough of their UP alignment to be forced back into an UP state.
In short, it looks like the the orientation of the external magnet field changes the orientation of the particles. Why is this evidence of superposition? Does it not simply suggest that spin is changeable?