This is used to distort your field of vision
cut out a disc, fold up the fan blades, make bearing at center of back with thumb tack, plastic bead and drinking straw. Rotate at slow even speed by blowing, stare at center for about 30 seconds then look at something else.
It's called the waterfall illusion, because you can get it looking at waterfalls.
This one looks magenta as long as you follow the circling dots, but turns green if you focus on the cross in the middle for a while.
I wonder if it would work if you made a paper one
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