Knowledge

Facts, science and history of optical illusions.
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Inferior and Superior Mirages, and Fata Morgana

Can animals see optical illusions? Hosico cat shows it with optical illusion rug

Trees wearing a tutu

Source/credit: Ohh_Hai_Dere

Trees wearing a tutu is obviously ridiculous. But it might not be immediately obvious what you are looking at in this illusion.
The tutus is actually sheets of ice stuck to trees after a flood. When the water receded, the ice remained suspended midair.