Images that are not what they seem on second glance, static objects that appear to shift suddenly, and visual cues that are not there at all—surely these must be the work of some strange, unseen force.

And, in fact, it is: it’s called your sense of vision.

Despite being one of the most basic ways in which we interpret the world, the system is by no means foolproof. Certain stimuli can fool the brain to the point of sending it into a delightfully entertaining tizzy, ranging from centuries-old paintings intending to deceive to inexplicable images such as “the dress” from 2015, which sparked endless debate after showing as different colors to different people. Explore the science behind optical illusions along with some top examples of them in action.

How do illusions work?

There is no single, straightforward answer to this query. Even scientists are perplexed by how exactly we fall for visual tricks and why their effects are so universal. In general, though, experts believe that this phenomenon occurs when the brain tries to interpret certain images and winds up misreading or altering them.

After all, vision isn’t quite as simple as just looking around. You could almost say that it’s like a game of telephone: at some point in the communication between your eyes and your mind, signals are bound to get crossed. That’s because your eyes don’t actually process images but merely absorb data from your surroundings. When light hits the retina in the back of each one, photoreceptors convert it into electric signals, which then travel through your optic nerve to your primary visual cortex. Here, your brain decodes these separate signals and shapes them into a single image, or what you see.

There are various ways in which this sense may make mistakes, leading to an optical illusion:

• Your mind tries and fails to comprehend ambiguous visual cues.
• It relies on past experiences and assumes that what you’re seeing is similar to other images you’ve encountered.
• It focuses on only certain parts of an image, modifying your interpretation of the entire picture.
• Where information or imagery seems missing, it fills in the blanks with elements that are not actually present.

To experience these effects for yourself, take a look at some popular illusions in action.

The Young Woman, Old Woman Ambiguous Figure

What do you see in the image above: an old woman or a young one? Are you able to toggle between them based on which part of the image you focus on? (If not, try to imagine the young maiden’s chin as a large nose or vice versa.) This late-1800s artwork is an example of a classic illusion type that has been popular for centuries due to its unusual effect on the viewer—though the image does not change, your interpretation of it does. Such pictures play with your brain’s ability to take static visual cues and refocus on certain details, prioritizing some over others to help you make sense of what you see.

There’s even an entire visual-arts movement known as trompe l’oeil (French for “to fool the eye”) that involves manipulating perspective to create images with believable three-dimensionality or startling imagery. For instance, an 1874 painting by Pere Borrell del Caso features a boy who looks to be climbing out of the frame. The renowned Salvador Dali also frequently leveraged this technique, including in his aptly titled Overture in Trompe l’Oeil.

Dizzying structure

Some illusions can perturb your sense of sight altogether, such as the one above. While the graphic may seem to be moving, it’s actually a still image. You can attribute this disorienting experience to the moiré effect, which causes you to interpret repeated patterns that converge to a point as one moving object rather than several still shapes. However, you can take some control over the illusion by focusing on just a small spot. The lines, at least in that area, should appear to hold still.

The checker-shadow illusion

Now consider an illusion known to fool some people but not others. The image above may seem fairly direct—the large black sphere casts its shadow onto a checkered surface, which features two squares labeled “X” that are different shades. But, believe it or not, both squares are the exact same color! Your mind could have trouble perceiving this because the supposedly lighter one is surrounded by darker squares and the image’s color pattern suggests a shadow.

This is a powerful example of an optical illusion that can utterly convince the brain that the contrasting shades are authentic—to the point that it practically resists seeing the image for what it really is. If you’re struggling to see both X squares as the same color, try covering part of the image with your hands to obscure the other information that leads to confusion.

The rubber pencil trick

In your school days, you or a classmate may have entertained fellow students with this widely known trick of the eye. Loosely hold a pencil near one end, and shake it rapidly up and down. Does it seem to wave as if it’s a soft object rather than a hard, straight one? There is a simple explanation for why this works: your brain just can’t keep up with how quickly the pencil moves as your eyes try to capture it in motion. As Patrick Pester, a trending news writer at Live Science, notes in an article on the topic, “Humans can only process 50 to 1,000 individual frames each second, so we only get a summary of what’s happening.”

When you look around the room you’re in, it’s easy to accept that every image makes perfect sense, even if you’re wearing glasses or contact lenses. In reality, though, the brain is an imperfect machine, and optical illusions are clear examples that, on occasion, certain objects warrant a second look.