Back in June we spotted a video that had us flummoxed: a few round-looking shapes appear as squares in a mirror. Rotating them 180 degrees produced the opposite result. Was it wizardry? CGI? Almost a month later, we finally have our answer.

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Captain Disillusion is a YouTuber best known for debunking viral hoaxes. But much to our—and potentially his—surprise, the ambiguous cylinder illusion isn’t the result of digital effects trickery. The secret lies in perspective cues and some very, very careful planning.

For starters, none of the shapes are truly round or square, they’re somewhere in between. So already your brain has to work a little harder to determine what it’s looking at. The truly clever bit is that the top surfaces of these shapes aren’t flat. Each is composed of rolling, s-shaped hills and valleys that, when viewed from the correct angle, have the effect of lengthening or shortening certain sides. In the original video you can almost make them out, now that you know what to look for.

But these tricks of construction aren’t enough on their own. Proper lighting has to be used to ensure that the shapes’ shadows don’t give the secret away. Even the camera’s specification have to be taken into account, because the exact dimensions of the shapes are determined in part by the focal length of the lens being used to film the trick.

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Would it have been easier with digital manipulation? No doubt. That or the illusion could’ve been shot in multiple takes. But knowing that this brain-bending trick is legit makes it all the more impressive.