Mueller-Lyer Illusion

The Mueller-Lyer Illusion is an optical illusion involving two lines and arrowheads at the ends of each line. When two lines and arrowheads of equal length are used, the one with ends pointing out is thought to be longer than the one with ends pointing in. See for yourself...

Which line looks longer (obviously they are the same length, but as you can see, they look to be different lengths)?

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