Prepare to have your brain befuddled this Friday morning by a simple* (*OK, infuriating) toy train track.
Two pieces, when laid on top of each other, are exactly the same size and shape. But when one is put beneath the other, it's suddenly so much longer.
It's a brilliant trick to use down the pub when your mates have had one too many fizzy drinks. Although the dramatic soundtrack really helps, which is Dies Irae and can be found on the iOS app Splice, apparently.
Thankfully, this is not 'the dress' all over again as there's already an explanation. Yes, while Twitter user Marc Blank-Settle sat back and enjoyed the unfolding mayhem, some were quick to point out he'd actually rediscovered the Jastrow Illusion.
American psychologist Joseph Jastrow is said to have first noticed this optical illusion in 1892. Basically, it's only if the ends are lined up together, as in the video, that one looks longer than the other. If you shift them so the middle of the arcs align they once again appear the same size. There's a handy drawing on Wikipedia to help us through the madness.