Why still rocks creep upward after you stare at a waterfall: the motion aftereffect
30seconds
Stare at a waterfall, then look at the rocks beside it, and they seem to crawl upward. They go nowhere, yet they move. This is the motion aftereffect, noticed by Aristotle and named by a Scottish scientist at the Falls of Foyers in 1834.
Try it on a screen: adapt to scrolling stripes, then watch frozen ones drift the wrong way. You will see how opposing motion detectors in your brain normally balance each other, and how tiring one of them tips the scale.
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