The strangest optical illusion of all

This is called – with variations – the Motion After Effect.  The reason that it happens is a wonderfully complex interaction between your brain and your eyes.  It was first observed by Aristotle, but was not fully described until the 19th century, and not understood until the 20th.

Stare at the spinning spiral for about thirty seconds, and then look at something static, such as the back of your hand.  Go on, do it now!  It probably won’t work if you’re viewing this post on a phone: it’s better full-screen.

Freaky, eh?  I once saw a children’s entertainer rig up one on stage.  He got the kids to stare at the circle as it spun around, and then look at the nose of the kid siting next to them.  It brought the house down.  He told me he hated the trick, because it required no skill or presentation, but that every time he took it out of the act, he was asked by clients to put it back in.

I also observed the phenomenon myself after a long ride facing backwards in a flat-bed truck in Venezuela (though I daresay it works in other South American countries as well). After several hours of staring at the retreating road, we stopped beneath some cliffs and they appeared to be leaning in towards me.