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, external on Wednesday evening, and been shared thousands of 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.
But - assuming that aliens aren't actually to blame - what causes the unusual effect? Others on Twitter were quick to point out the answer.
The Jastrow Illusion, external 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.
This image should help shed some light on the mystery.
Many of the people keen to explain the optical illusion said they remembered it being the first they were taught about in school, or even the first they had spotted when playing with similar toys as children.
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