One is dark, one is light. One is gray, one is white. These are obviously two different blocks with two different colors right? Wrong. The two blocks are the same color. Seriously, they're the same shade of gray. Just place your finger across the seam (where the blocks meet in the middle) to reveal the illusion. Just crazy.
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Seen this illusion before, but I think this one is presented poorly, because the blocks are actually not the same color, just the flat surface area is.
Personally I think this is a better presentation of this illusion. A and B are the same color.