An optical illusion has once again got the Internet scrambling for answers.

Remember the furore over The Dress?

Back in 2015, an image of a lacy bodycon dress was posted on Tumblr, and then spread like wildfire over social media as Internet users debated whether it was blue and black or white and gold.

This time, it is a group of six girls sitting on a sofa. But look closely and only five pairs of legs can be seen.

The image was first uploaded onto Reddit by user jr0d7771 on Christmas Eve, and has since been picked up by international media such as Esquire and the Daily Mail.

The original post has so far attracted more than 400 comments, with Reddit users discussing and attempting to figure out where the last pair of legs are.

Some, like user VintageErk, went out on a limb suggesting one of the girls must have been "man-spreading", a term used to describe when a man sits with his legs wide apart.

Others guessed that the answer was simply what it was - that one of the girls does not have legs.

But some users seemed to be onto something, such as Redditor justonewordforyou, who suggested that increasing the brightness of the picture would help one identify the lines demarcating the girls' bodies.

It seems most plausible that the mix-up is between the first two girls on the left.

The first girl is wearing black jeans. The second girl is wearing black jeans too, but they are ripped at the knees.

So what seems like the first girl's right leg crossing over her left leg, is actually her leg and the second girl's leg stuck together.

That leaves the third girl with blue jeans, and the rest matches up.

Still can't see it? Try making your screen brighter and take another look.