Is a dress blue and black or white and gold? That is the question that has people around the world scratching their heads and rubbing their eyes in disbelief.
Seeing is believing, but in this case, the vote was almost evenly split on what many people take to be uncontentious - the colour of a piece of clothing.
Some people apparently see it as one colour combination at first, then another a few minutes later.
The online brouhaha began with a Tumblr post of a lacy bodycon dress with a plea: "guys please help me - is this dress white and gold, or blue and black? Me and my friends can't agree and we are freaking... out".
That photo, posted on Thursday, had more than 300,000 likes on the social media site on Friday morning. It was posted by singer Caitlin McNeill, 21 from Scotland, news site Business Insider reported.
The debate over the colour of #TheDress also caught the attention of Singapore Prime Minister Lee Hsien Loong, who pointed out that it was more than an internet craze. He wrote in a Facebook post on Friday: "But unlike some other internet crazes, this one has a scientific point. We think of the colour of things as being something fixed, a property of the thing."
He added: "But in fact what colour we see depends on the context in which we see the thing, and on how our brains process the light which enters our eyes. So different people will sometimes see the same thing as having different colours."
By the way, Mr Lee saw white and gold.
Ms McNeill, who posted the original picture, said: "What happened was two of my close friends were actually getting married and the mother of the bride took a photo of the dress to send to her daughter." She added: "When my friend showed the dress to her fiancee, they disagreed on the color."
Ms McNeill first put up the question on her Facebook page, but when all of her friends could not agree on the colour, she took the post to Tumblr, where she maintains a fan page dedicated to a woman named Sarah Weichel.
Ms Weichel is a talent manager who represents several YouTubers. The post went viral after that. Ms McNeill said that she never imagined that her post would be tweeted about by stars like Taylor Swift and Mindy Kaling.
The online discussion also generated comments like: "I see it as white and gold. My friend right here sees it as blue and black. I CANT HANDLE THIS", and "My class just had a debate over this. Half sees black and blue, the other half sees gold and white. Someone please explain this."
It has also spawned a number of hashtags - #dresscolour, #thedress,#whiteandgold, #blueandblack - and articles on numerous sites.
Quite a few explanations of how the discrepancy in perception can occur have been circulating.
Our eyes perceive colours based on different wavelengths that reflect off what we see. Mr Jay Neitz, a neuroscientist at the University of Washington, told Wired that the image of the dress hits a "perception boundary".
The disparity is due to people's eyes compensating for the lighting in the photo in different ways.
"In the case of the dress, some people are deciding that there is a fair amount of illumination on a blue and black (or less reflective) dress. Other people are deciding that it is less illumination on a white/gold dress (it is in shadow, but more reflective)," Mr Cedar Riener, associate professor of psychology at Randolph-Macon College explained on Buzzfeed.
For the record, the dress is blue and black, according to people who saw it firsthand at a wedding in Scotland, and according to Ms McNeill as well.
The dress is sold by British fashion retailer Roman Originals, and on its website it is called the "royal-blue lace detail bodycon dress".
About a week before the wedding, the mother of the bride sent a photo to the couple of what she planned to wear. The bride and groom couldn't decide if the dress was blue and black or white and gold, so they posted the photo on Facebook.
What colour do you see it as?