One has four corners and the other has none, but it's baffling to tell the difference between a circle and a square in this simple illusion created by Kokichi Sugihara of Japan's Meiji University. Taking second place in the Neural Correlate Society's Illusion of the Year contest, Sugihara's video demonstration of shapes that look like squares from the front and circles from the back baffled Reddit over the holiday weekend. It's called the "Ambiguous Cylinder Optical Illusion," and you can check it out below.
Jason Kottke found a video by Make Anything that reverse-engineers the illusion, and explains how it works.
Last year Sugihara also won 2nd place in the Illusion of the Year contest for an ambiguously-shaped miniature garage roof.
Illusions have been used en force by artists big and small, ranging from the Op Artists of the 60's to Nonotak's recent mind-melting Hoshi installation at The Creators Project's Future Forward event series and street artist JR cloaking the Pyramide du Louvre with a perfect image of the museum behind it. See more illusions on the Illusion of the Year website.