Missing (or Extra) Square Optical Illusion

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Four pieces of a square are rearranged to create a rectangle that appears to have a different area than the original square. Rearranging the pieces with a second layout forms three rectangles that also seem to have a different total area. The change in area is caused by a slight overlap or gap along a diagonal that becomes more apparent with smaller pieces.
Contributed by: Henry Huang (March 2011)
Open content licensed under CC BY-NC-SA
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"Missing (or Extra) Square Optical Illusion"
http://demonstrations.wolfram.com/MissingOrExtraSquareOpticalIllusion/
Wolfram Demonstrations Project
Published: March 7 2011