Every now and then an image appears online which has people heartily debating its authenticity, arguing about its colour or asking how it came about in the first place.
There are also some amazing pictures that look like one thing but turn out to be something quite different.
We've gathered together a collection of the best from around the 'net - pictures that'll have you scratching your head, questioning your eyesight and generally feeling perplexed by their existence.
White and gold or black and blue? That dress
Back in the hazy days of 2015, a certain dress took the internet by storm and had everyone debating about whether it was white and gold or black and blue.
The simple garment was the perfect example of an optical illusion that was no illusion at all. It was, in fact, just a scientific demonstration of how the eye sees things differently in varying light sources and how our brains all handle this interpretation in ways that are unique to us.
The non-red strawberries
Like the white and gold/black and blue dress, these strawberries are an optical illusion caused by something known as colour constancy, which is a feature of the way we perceive colours under varying light conditions.
This picture has NO red pixels. Great demo of color constancy (ht Akiyoshi Kitaoka) pic.twitter.com/pZHvbB6QHE— Matt Lieberman (@social_brains) 27 February 2017
There is no red in the picture and yet our brains clearly interpret the red of the strawberries as still being there – assuming you're seeing them under a blue light source and extrapolating accordingly.
The shiny legs
This is an unusual illusion, at first glance you'll probably spy a pair legs that seem to be covered in oil, but given a second to look again and you'll see they're actually spattered with white paint.
are these legs shiny and oily or are they legs with white paint on them pic.twitter.com/7Z8e8F1JCZ— kayden (@kingkayden) 26 October 2016
The car seat/iPad conundrum
Sometimes an illusion can be as simple as a pattern blended into another in the same way that camouflage works.
This photo was posted on 9gag with the headline "Just spent 30 mins searching for my Dad's iPad" – can you spot the hidden tablet in the photo?
The four-eyed man
As far as optical illusions go, this one is both simple and painful. Whoever thought two pairs of eyes and two mouths would be enough to break your brain and make it painful to focus?
The infinite chocolate bar
This tasty illusion apparently shows a way to get never-ending blocks of chocolate out of a single bar. Alas, it's all a trick and simply involves a shorter and shorter bar. Still pretty clever stuff!
The unnaturally long train track
In another Tweet-storm, yet another optical illusion based on an everyday object appears to confound logic and physics. One wooden train track block is apparently longer than the other, yet when put on top of the other, they're the same length. How?
My toddler's train track is freaking me out right now. What is going on here?! pic.twitter.com/9o8bVWF5KO— marc blank-settle (@MarcSettle) 6 April 2016
The solution to this apparent mystery is contained within the replies to the initial Tweet, but we'll let you enjoy it in the meantime.
The negative lady
This illusion requires you to stare at the white dot on the woman's nose on the left for 15 seconds, then look to the right of the image at the blank space. You should see a flicker of the full colour photo of the woman.
This is down to the way our brains interpret imagery and colours and in this case is known as "negative afterimage", which was explained by Dr Juno Kim from the University of NSW School of Optometry and Vision Science to Daily Mail Australia:
"The code for all the hues we can experience in the light spectrum – this information is relayed from the back of the eye to the brain via three opponent neuron channels.
"When you look at something that, for example, is yellow for a long period of time, you stimulate the cells that are positively sensitive to yellow – so in the yellow and blue channel.
"The cells’ activity increases and after a bit of time the activity fatigues and declines.
"When you then direct your gaze at a uniform background – let’s say a grey wall – then what happens is that the cell doesn’t return to its resting activity, it goes much lower than that.
"It’s that decline – the weakening of the yellow code – that codes for the opposite colour to become stronger, so you’ll see blue."
Underwater jumping girl
This photo appeared on Imgur and caused a bit of a splash. A little girl appears to be jumping into a swimming pool while simultaneously blowing bubbles like she's already underneath the surface.
Commenters quickly pointed out that her hair is dry and the apparent air bubbles could just be droplets of water from the splashing, but no conclusion was reached as to what was actually happening.
The revolving snakes
This one is a simple trick of the eye. This is not an animated picture, it's a static file that shows a mass of intertwined snakes. But if you stare at different sections you'll see the snakes writhing and squirming.
The spinning dancer
Some have suggested that this dancer will spin in different directions depending on what side of your brain you're using.
In fact it's just down to how your brain interprets the picture and you can see this in action by splitting the image into two versions. Just cover each part with your hand to see it spin the other way.
At first glance you might see the dancer spin clockwise and in the next moment the opposite direction.
Duck or rabbit?
This optical illusion has taken on many different forms since it first found its way into publication in a book "Philosophical investigations" by Ludwig Wittgenstein in 1953.
Here it is suggested that there are ambiguous images that can be seen in two (or more) different ways.
In this instance, the drawing could be a rabbit or a duck or both and what you see first will be down to your perception of the world or based solely on suggestion - the duck and rabbit axis in this version make it easier to quickly decipher both variations.
This photograph taken on the beach seems to show the speaker on a floating platform. No doubt down to our brain interpreting the shadow of the flag blowing in the wind as the shadow of the speaker's platform and microphone instead.
Another classic illusion caused by our brains trying to decipher the scene before us.
There are a few of these sorts of optical illusions on the web – a simple image of two people embracing throws your brain into confusion where the couple are at strange angles and it's hard to immediately decipher which head (or other body part) belongs to which person.
Here Stuart Rutherford managed to magic an owl's face inside his coffee mug by simply dropping a couple of Hula Hoops into the mix.
Who'd a thunk dunking a couple of Hula Hoops in your coffee would be so beautiful pic.twitter.com/lsTgnvUc— Stuart Rutherford (@doodlewhale) 26 September 2012
Ambiguous cylinder illusion
One of the finalists for the "Best Illusion of the Year Contest" makes for pretty mind-bending viewing.
Here Kokichi Sugihara places a set of cylinders in front of a mirror. The reflection shows a different shape until the object is revolved and then we see the opposite. It's best viewed in the video below.
If you're left scratching your head after watching that video, you can see a breakdown of how it works here:
The magic eye shark
In the 1990's, Magic Eye was an extremely popular series of books which allowed people to see 3D images by focussing on an otherwise 2D image – usually in the form of a non-descriptive pattern.
Last year Blake Lively posted an image to her Instagram account showing a Magic Eye-like image to help promote her shark movie "The Shallows".
If you're struggling, the official Magic Eye website has instructions on how to view the 3D images but basically you're trying to focus through the image while looking at it (or blurring your vision until it becomes clear).
Common sense crossing
Erik Johansson is a Swedish photographer and a whizz with Photoshop who likes to take real photographs and turn them into surreal optical illusions.
Common sense crossing is just one of his many works and one that we find particularly messes with our eyes!
Helicopter takes off without moving
At first glance you'd be forgiven for thinking that this helicopter was using some sort of new military stealth technology to take off without spinning its propellers.
In fact it's just an illusion created by matching the video camera's shutter speed to the rotation speed of the rotors.
Magnificent artist Howard Lee creates hyper-realistic drawings that are so brilliant it's hard to tell them from the real thing.
This talent for creating optical illusions is demonstrated in his Youtube video which shows him cutting, bashing and setting fire to the real versions of his creations.
Stefan Pabst is yet another talented artist with the ability to create optical illusions from drawings, paintings and sketches. In a series of works he creates the illusion of three-dimensional images on flat paper surfaces.
Warning, contains spiders.
Pizza delivery man
Every now and then advertising companies do something clever that captures our imagination. Something as simple an optical illusion can be enough to help a brand stick in our minds.
This advert from Papa John's pizza was deemed clever enough to win an award at the Cannes International Advertising Awards and simply featured a mini pizza delivery man on a bit of card that could be put on someone's front door so when they looked through the viewing hole they saw the man on the other side.
Making the impossible seem plausible
In another advertising campaign, Honda produced a video featuring a range of optical illusions to help sell their new CR-V – all aimed at helping to promote a big car experience but with a smaller economy vehicle.
Incredible street artist Sergio Odeith turns everyday corners in the real world into incredible three dimensional artworks with ultra-realistic anamorphic graffiti painted across different surfaces to give the illusion of a depth that doesn't exist.
Letters leaping out of walls, creatures lurking in corners, other worlds coming to life in the corner of a forgotten space. His works play havoc with the eye and create marvellous murals that are a pleasure to behold.
The bridge to nowhere
The Storseisundet bridge is one of the largest bridges in Norway and photograph from the right angle it gives the impression of being a bit of road that comes to an abrupt and dangerous end some 75 feet above the ground.
However, the bridge is actually a cantilever design with a curve to it that simply makes it arc away into the distance.
The flying weightlifter
Lesman Paredes was pretty happy with his performance at the Junior Weightlifting Championships in 2015 so much so that after completing a 230kg clean and jerk he can be seen taking off from the ground.
Alas, this is not the case of Superman finally revealing himself to the world, but simply a combination of bouncy surface underfoot, heavy weights being dropped after a successful lift and a well-timed super celebratory jump. Unsurprisingly, the photo was later subjected to a Reddit photoshop battle.
You can see it all in action in this video:
Anamorphic advertising artwork
In 2013, eyewear manufacturer Ray-Ban teamed up with optical illusion masters Brusspup to create an ad campaign which included anamorphic illustrations which look like real three-dimensional objects from the right angle.
The globe and baseball are the standout pieces of this artwork and we think they're thoroughly impressive. For more optical illusion goodness, check out Brusspup's Youtube channel.