Muller-Lyer Illusions

The Muller-Lyer illusion is an optical illusion in which two lines of equal length appear to be unequal depending on which direction the arrowheads at the ends of each line point. When the arrowheads point away from each other the line appears shorter. When the arrowheads point toward each other, the line appears longer. The illusion was first discovered in 1889 by German sociologist Franz Carl Müller-Lyer.

There are different theories which attempt to explain the phenomenon. I think this article from The Illusion Index provides a nice summary. 

While the Muller-Lyer illusion is mildly interesting, the effect becomes mind blowing when movement is added to the arrowheads. Here are examples of Gianni A. Sarcone's dynamic Muller-Lyer illusions.