Müller-Lyer Illusion

What: The Müller-Lyer illusion (pronounced roughly MEW-ler LEER) is one of the absolute classics, you’ve probably seen it in a textbook at some point. Two lines, exactly the same length, one with arrowheads pointing out and one with tails pointing in… and the one with the tails looks noticeably longer. Here they’re animated, so the fins swing back and forth and you can watch the lines seemingly grow and shrink, even though they never change at all.