The spokes of this wheel
are not turning.
Only the shades of grey are changing.
Why does this work?
Another cool illusion is the "reverse spokes" illusion, which has been
studied by Anstis and Rogers (and probably discovered as well. Anstis
describes the illusion very modestly as "new, or newish".). …the spokes (the grey lines separating the
"pie-slices") appear to be rotating clockwise. On closer inspection you
can clearly see that this is not the case. In fact, what is happening is
that a "colour wave" traverses the wheel in a counterclockwise
direction. That’s weird.
For more explanation, see Reverse spokes at CogSci.nl, by Sebastiaan Mathot