A photo of some hikers clinking some whiskey bottles has left viewers scratching their heads of the number of people featured in the snap.
The image shows the hikers clinking miniature bottles of bourbon to celebrate completing the 2,200 mile Appalachian Train in the United States.
How many arms do you see?
A total of four bottles can be seen in the picture, but at first glance it appears that only three arms are outstretched.
The picture, shared by Parick Delvernois who is wearing the kilt and orange jacket, tricks viewers into only seeing three people, as one of the hikers is wearing a camouflage jacket which blends perfectly into the background.
On closer inspection, you can spot the fourth arm outstretched on the left of the photo.
The freaky optical illusion, which was shared on Reddit and later on Twitter, has left the internet baffled, with some saying it left them confused and that their brain refused to believe four people were featured in the snap.
One user said: “I’m confused... why are there only 3 arms, yet 4 bottles?"
Another said: "My brain refuses to believe there are 4 people in this photo."
A third added: “This is the first time I’ve ever seen camouflage worn effectively on the internet and my mind is blown.”
Someone else said: "Me looking at this photo asking myself if I’m blind or if there’s something more going on here."
Some users questioned whether a person was standing behind Patrick due to the shape behind him and wondered if this was the mystery fourth person.
However, the ‘missing arm’ is actually on the left of the image and can be spotted if you look closely at the hands.
It is possible to make out the shape of a blue woolly glove, although it is not immediately obvious as the hand lines up almost perfectly with Patrick’s kilt.
One anguished user commented: "I zoomed in, stared at those blue fingers for a good 30 seconds trying to figure it out.
"Multiple solutions went through my head. Maybe it’s a child underneath the cluster of hands.
"Maybe somebody is standing behind the guy in orange. Then I saw it."
Another person added: “Yeah took me a solid 10 seconds until I saw the arm. Good camo.”
A third said: “I’m embarrassed by how long it took me.”
Patrick, 23, from Pittsburgh, Pennsylvania, told the Metro: “I actually saw this picture for the first time when my friends sent me a screenshot of the Reddit post.
“At the time of the photos being captured, I, as well as my friends, were unaware of the optical illusion that was present.”