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It's cool, you can turn on zooming (option-command-8) and then zoom in on one of the smaller ones (option-command- +) and watch it change as you zoom in. Then zoom out (option-command- - ) and watch it change back.

Very cool illusion.
 
They say it's originally from a paper on face-perception. Really interesting stuff, that is. You can see how strange our mechanisms for perceiving faces are from the illusion...
 
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