Illusion

your eyes have opinions. rulers don’t.

Müller-Lyer and friends — vote, then measure

Both pairs of lines on this card are exactly the same length — a ruler would agree — but the fins bend your read of depth, and a vertical stroke reads longer than an identical horizontal one to most people. Knowing the trick doesn’t make it stop working, which is what separates it from a mistake you could correct by trying harder.

Concepts: perception, optical-illusions.

An interactive you play with one thumb, on Wundr.