Optical illusions are popular again thanks to the internet. (Perhaps there's some irony to the renewed interest in these visual tricks: the most common illusions rely on people not looking too closely — and not looking too closely is one of the main side effects of internet-based media consumption.)

Whether you see optical illusions as neat party tricks or examples of the complexities of human perception and the evolution of the brain, they're undeniably fascinating. Here are 10 classic optical illusions and the reasons why they continue to fool people to this day.

1. The Spinning Dancer Illusion

The spinning dancer illusion Is this figure spinning clockwise or counter-clockwise? (Image: Nobuyuki Kayahara/Wikimedia Commons)

This image shows a female dancer pirouetting in a continuous loop. The illusion has to do with the direction in which she is spinning. Viewers will initially think that she is turning in either a clockwise or counterclockwise direction. However, when observing the image at other times, the same viewer may feel absolutely certain that the dancer is spinning in the opposite direction. In reality, this animation is on a continuous loop and does not suddenly change directions.

Why is it impossible to define the direction of the spin? This optical illusion tricks you with depth perception. Because there aren't enough clues about the depth of the dancer image, no one can define the direction of her spin with absolute certainty. The image is a silhouette, so the dancer’s pony tail or extended arm can only be seen when she's facing to the side. They disappear from view at the critical moment when we need them to provide information about depth and direction.

2. Ames Room

The Ames Room is one of the best (and most famous) examples of how perception is not always the same as the reality of what's in front of them. When viewed from the front, an Ames Room appears to be a normal, square room with walls perpendicular to floor and ceiling. However, the back wall is in fact built at a sharp angle and the floor and ceiling are steeply slanted.

This creates an illusion that makes people and objects on one side of the room seem much smaller or larger than people or objects on the other side of the room. Since perspective is important, many Ames Room models have some sort of pinhole viewing device so that the viewer sees the room from an angle where they cannot perceive the slanted floor and the different lengths of the walls. The idea behind the Ames Room has been used in movies such as "The Lord of the Rings" series to make some actors seem much shorter or taller than they really are.

3. Cafe Wall Illusion

An illustration of the Cafe Wall Illusion The Cafe Wall Illusion actually appears on a cafe wall in Bristol, England. (Photo: Peter Hermes Furian/Shutterstock)

The so-called Cafe Wall Illusion has been described as a checkerboard with the squares slightly jumbled or off-kilter. The alternating light and dark squares do not line up directly with the squares on the rows above and below them. The result is that the horizontal lines in between each row appear to be slanted. In reality, the horizontal lines are perfectly parallel with one another and totally straight.

The Cafe Wall Illusion (named after a wall in a Bristol cafe that inspired the idea) is similar to the Zollner Illusion, in which short line patterns are drawn across longer parallel lines at different angles to make the longer lines seem slanted when, in fact, they are perfectly straight. Most theories for explaining why the eye is fooled by such patterns have to do with people first focusing on the contrast between the light and dark spaces. They subconsciously use this contrast to define all the other shapes in the image in their mind even before they have focused on all the parts of the picture.

4. Simultaneous Contrast Illusion

The Simultaneous Contrast Illusion The Simultaneous Contrast Illusion uses a shaded background to trick the viewer into inferring things about the color of the main object (Photo: Dodek/Wikimedia Commons)

In this image, the horizontal bar in the middle of the picture is one solid color. However, the changing gradient behind the bar makes it seem like the color of the bar itself is changing as it goes from left to right. If the viewer uses their hands to block the background, it becomes obvious that the whole bar is indeed only one shade of gray.

This trick shows that people tend to make assumptions based on surroundings rather than looking only at a singular object. This may seem like a flaw in humans' abilities of perception (the inability to look at an object without being influenced by the background), but it could also be seen as a positive trait (being able to look at the "big picture").

5. Penrose Stairs

An illustration of Penrose Stairs The Penrose Stairs, sometimes called impossible stairs, were created by father-and-son team Lionel and Roger Penrose (Photo: Sakurambo/Wikimedia Commons)

The Penrose Stairs, also known as the Impossible Staircase, is perhaps one of the most famous of all optical illusions. In this two dimensional image, the four flights of stairs appear to link together so that a climber would go up or down the steps in a continuous loop but never arrive at a higher or lower point.

Though it can only exist in two dimensions, the Penrose Stairs have become a popular subject for artists, who depict the stairs and other similar distorted perspectives in two dimensional paintings or drawings. In fact, this is just one example of the kind of perspective tricks that painters and other artists working with two-dimensional mediums have employed for centuries.

6. Ponzo Illusion

 Ponzo Illusion illustration The Ponzo Illusion is named after Italian psychologist Mario Ponzo, who came up with it in 1911. (Photo: Timjarrett/Wikimedia Commons)

The Ponzo Illusion relies on geometric shapes to trick the eye. The idea is similar to the Simultaneous Contrast Illusion: People will make assumptions about an object based on the information that they get from the background.

The illusion is very easy to recreate. When he first came up with it, Italian psychologist Mario Ponzo drew two horizontal parallel lines that were exactly the same length. By then drawing vertical lines that gradually got closer together (not unlike a picture of a railroad going off into the horizon), Ponzo was able to trick viewers into thinking that the parallel line in the background was much longer than the one in the foreground.

7. Necker Cube

An illustration of a Necker Cube The Necker Cube is an example of an ambiguous image. (Photo: BenFrantzDale/Wikimedia Commons)

The Necker Cube is a drawing of a square that has no clues about depth. This means that it's virtually impossible to decide which part of the square is the front and which is the back. The idea is named after a Swiss scientist, Louis Necker, who first drew the square. The basic illusion is that some people will perceive a three dimensional cube with one side in the front while others will imagine that the very same side is the back of the cube.

The Necker Cube can quickly become the subject of an argument, with some viewers having a strong opinion about which side is which. So often the challenge is not to decide which side is the front and which is the back, but to try and see the cube from another perspective (where the side that you're certain is the front is actually in the back).

8. The Rabbit Duck Head

Rabbit Duck Head illustration This 'reversible image' first appeared in a German humor magazine. (Photo: Wikimedia Commons)

One of the earliest examples of an ambiguous illustration, the Rabbit-Duck Head Illusion was first published in Germany in the late 19th century. Such hidden images were quite popular at the time. In this particular example, the duck's beak becomes the rabbit’s ears and the eye is drawn in such a way that it fits for both the right-facing and left-facing animals.

The image was used by early 20th century philosopher Ludwig Wittgenstein, who studied the idea of perception and wrote academic papers that discussed the ways in which people perceive things.

9. The Ternus Illusion

Ternus display Joseph Ternus is credited with coming up with the idea that is the basis for animated signage all over the world. (Photo: GregHubbard/Wikimedia Commons)

The Ternus Effect uses different frames of the same images to create a sense of motion. The idea was first discussed by psychologist Joseph Ternus. He came up with what seems like a pretty simple idea: If you show people the same image in different locations, they will automatically assume motion.

This idea is the basis for animation and for the kind of animated signage that's so common today. Because people are familiar with this kind of faux motion, it might not seem like an illusion. In fact, the Ternus Effect has been the subject of more research than any of the other illusions we have mentioned, with new studies and hypothesis being tested as recently as the early 2010s.

10. Optical art

Optical art uses elaborate color patterns to make static images appear animated Optical art uses elaborate color patterns to make static images appear animated (Photo: Fiestoforo/Wikimedia Commons)

Optical art images seem to be moving even though they aren't animated. Most theories about the illusion of motion in optical art have to do with the brain’s inability to process the different colors and shapes simultaneously. In short, there is so much going on in these pictures that motion is perceived where none really exists.

These images are most often associated with psychedelic art, but other schools also use optical art techniques, which have their roots in impressionism, dadaism, cubism and other more-classic forms of art. Many artists see this style as a way to challenge the perception of viewers or get them to interact with the work rather than simply viewing it.