Is this gold mug round or square? This is the latest illusion question boiling away online – with comparisons to the blue dress/black dress puzzle that had the world tearing its hair out a short while back.

British puzzle maker Tony Fisher (famous for building a Rubik’s cube as big as a cubby house) has created a gold mug that looks to be both round and square, and people are furiously hub-bubbing: which one is it?

In a video posted to YouTube, the cup appears to change shape as Mr Fisher moves it about in his hands. He sets the apparently round mug in front of a mirror where uncannily it appears to be a different object entirely: square-shaped. He then pulls a classic magician’s stunt of misdirection by filling the mug with milk and giving it a stir with a spoon. He does this, he says, to prove the mug is no trick of camera foolery.

This prompted a raft of comments on social media about how puzzling it all was.

In fact, the mirror and the milk give the truth away about what’s really going on.

The New Daily sent the video clip and some published photographs of the mug to Andrew Metha, Associate Professor and Deputy Director of the Melbourne Neuroscience Institute, Department of Optometry and Vision Sciences, University of Melbourne.

Dr Metha has written lectures on geometrical illusions for a new course called Art, Illusion and Deception – and he has lectured on illusion to his optometry students.

“The gold cup is a neat illusion, but it’s not in the same class as the blue or black dress, or the auditory Yanny and Laurel one. They were interesting because they showed something called perceptual relativity, where two people can look at exactly the same thing at the same time and come to different conclusions about what they’ve perceived,” he said.

“The gold cup is more of a standard geometrical illusion.”

The cup’s true shape is best seen from this angle. Photo: YouTube

The cup, when held at an angle, is shown to have a wave-like rim – and is neither purely round or square.

“It’s not a normal cup,” Dr Metha said.

“It’s designed to look like round when viewed from one direction … and square when viewed from another direction. It’s not like two people would look at the same thing and come to different conclusions. In fact, when viewed at a different angle it just looks weird.”

Dr Metha said the gold cup had been “sensationalised” to ride off the back of the blue/black dress sensation.

“Nonetheless it’s very cool.”

Dr Metha pointed out that Mr Fisher’s gold cup is a variation of round/square illusions that have been staged for some years by Japanese engineering professor Dr Sugihara Kokichi.

The Japanese professor has created 3-D illusions that mimic the impossible effects found in the famous Escher woodcuts of stairs going nowhere and water flowing uphill. He created an even more mind-blowing version of the gold cup trick using plastic cylinders for the 2016 Illusion of the Year competition.

In both cases, a mirror was used – and this allowed the viewer to see the cup or the cylinder from both angles. Cold logic should tell us the objects are in fact two-sided, but our perceptions are fooled into thinking something magical is happening.

“That’s the secret,” Dr Metha said. “It’s as boring as that.”