A tweet by the BBC’s Marc Blank-Settle about his kid’s train track has been baffling the Internet. In the video you can see the two track pieces are the same size when placed on top of each other. However, once you place one beneath the other, the bottom appears noticeably larger.

So what exactly is going on?

The trick of the mind is known as the “Jastrow illusion,” named after the American psychologist who first explained the illusion in a study in 1892.

In Jastrow’s words: “In judging areas, we cannot avoid taking into account the lengths of the lines by which the areas are limited, a contrast in the lengths of these is carried over to the comparison of the areas.”

Essentially, we’re measuring the outer arc of the bottom piece against the inner arc of the top piece. In our minds, we make a judgement on the size by simply comparing the inner arc length to the outer arc length. However the outer arc (on both pieces) will always be longer than the inner. If the tracks are moved to a more “appropriate” place, either on top of each other or not aligned at their edges, you won’t inadvertently make comparisons between the inner and outer arcs and you’ll realize they’re actually the same size.

My toddler's train track is freaking me out right now. What is going on here?! pic.twitter.com/9o8bVWF5KO

— marc blank-settle (@MarcSettle) 6 April 2016