A perplexing video of what looks like a series of shape-shifting objects has been watched more than two million times on YouTube as internet users struggle to solve the puzzle.
The "ambiguous cylinder illusion" was created by artist Kokichi Sugihara and won him a finalist place in the Best Illusion of the Year Contest 2016.
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"The direct views of the objects and their mirror images generate quite different interpretations of the 3D shapes," he says.
"We cannot correct our interpretations, although we logically know that they come from the same objects. Even if the object is rotated in front of a viewer, it is difficult to understand the true shape of the object and thus the illusion does not disappear."
The puzzle finished second in the contest, which was held by the Neural Correlate Society, an organisation that "promotes scientific research into the neural correlates of perception and cognition", reports the Daily Telegraph.
A few enthusiasts with a 3D printer believe they have solved the riddle, saying the shapes in the original illusion are "squircles" - half square, half circle. Two of the sides arch upwards, and the other two dip downwards, which makes the "squircle" look like either a square or a circle depending on the angle you are viewing it from.
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