train track illusion
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The train track optical illusion that's bewildering the internet

This video - of a father shuffling around his son's toy train tracks - has bewildered the internet. Can you figure out what's causing the optical illusion?

It was posted on Twitter by BBC trainer Marc Blank-Settle on Wednesday evening, and been shared more than 3,000 times since then.

The track on the bottom appears to be much longer than the one on the top, but when one is placed on top of the other, its clear that they are in fact the same size.

Many viewers were left completely dumbfounded.

The Jastrow Illusion is an optical illusion first defined by an American psychologist, Joseph Jastrow, more than 100 years ago. It involves two arc shapes of equal size, where the longer side of one arc is brought into contrast with the shorter side of the other.

When the images are arranged as they are in Marc's video, the top left corner of the lower piece is actually shifted over to the right, to align with the bottom left corner of the piece on top.