I have always been intrigued by optical illusions, seeing them as providing fascinating insights into how the brain works as well as warnings that what we think we see may not accurately represent what is actually there. I found this illusion (via David Pescovitz) to be particularly intriguing because the contradiction is so stark. You know that the four horizontal blue lines must be parallel because the background black and grey objects are all the same size, and yet to my eyes the top and third line unmistakably slope up to the right while the second and bottom lines slope down.
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