When we stare at optical illusions, we think we know what we see, and then, well, the image changes. What are they doing? Showing the strangeness of reality? Or just tricking the mind?

Optical illusions have been the subject of many scientific studies. And for good reason: They can give us great insight into how our brains work. They can also tell doctors a lot about some medical conditions, such as migraine. The truth is that our brains are hardwired to see things in a certain way. Optical illusions challenge this wiring. In other words, when you look at an optical illusion, you brain is confused and misinterprets the image.

Trick your brain and test your perception with these 7 clever optical illusions, and find out how they work.

Checker shadow

Check out the squares marked "1" and "2." One is light and one is dark, right? Drag the gray shape on the right between squares 1 and 2. Now what do you see?

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In this illusion, the shadow from the skull tricks your brain into thinking squares 1 and 2 are different shades of gray, in part because there appear to be darker squares next to square 1. In reality, your brain can’t tell the difference. When you slide the gray shape on the right between the squares, you find out the two squares are actually the same color.

Gray squares

Look at this series of squares. Can you see the gray squares at the intersections of the white lines?

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Bright objects look brighter when viewed against a dark background. In this example, you can see that the white lines seem brighter next to the dark ones. So the intersections of the white lines, where white is bordered with white, look less bright, or gray.

Now, try looking directly at one of these gray squares. Why do they disappear? This is because they are seen only by rods, cells in your eye that see black and white and give you peripheral vision. The rods are located only on the edges of your retina, while the cones, cells that see color, are located in the center. When you try to look directly at the gray squares, they disappear because you cannot see them with the cones in your eye (center vision).

Spiraling design

In this illusion, the design spirals in toward the center. Or does it? Click "Show alternating color" to find out.

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When you add more color, it is easier to see that the design is not a spiral, but actually a series of unconnected circles. Before you add another color, your brain interprets this image as a deep tunnel, and as you look into its depths, the overlap and arrangement of the circular lines makes the illusion of a spiral. In other words, your brain jumps to the wrong conclusion.

Bicyclists

In the picture below, the two men seem fairly close in size, although the man in back is a little smaller.

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Now, move the men around with your mouse. When you place the men next to each together, you can see that their size is not as close as it may have appeared at first. The man in back appears to shrink.

This dramatically illustrates how perspective makes our brains think that two different-sized objects can appear to be about the same size.

Vase or faces?

Do you see a vase or two faces in this picture? To see both, click the buttons.

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At first look, you may see the vase, or the faces, but usually not both. That’s because your brain tends to organize the picture into a main subject plus a background. Once you see it one way, it is harder to see it the other way.

Spinning skeleton

Look at the skeleton. Your brain can see it spinning to the left or right, although at first you may be able to see it spinning in only one direction. Click the Right and Left buttons to make the image spin the other way. Look away, then look back. Does the image spin in a different direction? Can you do it without clicking a button?

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This illusion is based on one called The Spinning Lady. Clicking a button can help you see the skeleton spin in one direction. But you may also be able to make the image spin in a different direction by looking away and then quickly back. Some (unscientific) online examples claim the dominant spin of the image can tell you whether you use more of the right side of your brain (creativity) or the left side (logic), which is untrue. Truth is, your perception determines which way the skeleton spins (if we weren’t forcing you to click directional buttons).

Flag stare

Stare at the flag for 40 seconds. Then, without moving your eyes, click the "Hide Flag" button to see the flag in its natural colors. Click the "Show Flag" button to bring the image back.

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The unusual flag you looked at first was composed of the complementary colors of the actual flag. When complementary colors are added together they make white.

Red and aqua are complementary colors, for example. The eye, after looking at the aqua stripes on the first flag for a while, becomes tired of recording that color. When you change your view to a white surface, the eye records the white with the aqua subtracted. So you see red. This is called an afterimage.

Necker cubes

As the lines in the spinning cubes cross, do you see the front of each cube or the back? Click "Show flat shaded color." Does it help your eyes clarify the images?

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The reality is that you can see both the front and the back of the cube. The lines of the images are drawn from what’s called isometric perspective. That is, the parallel edges of the cube are drawn as parallel lines. When you stare at the picture, the crossed lines in the image will seem to flip back and forth: Now you see the front angle; now you see the back. This illusion is called multistable perception.