A vision exam is one of the easiest tests you’ll ever take in your life—or, at least, it should be. All you have to do is cover one eye at a time and look at a chart! It might seem simple, but some eyesight tests are far trickier than others…

For instance, a large number of people who try these particular vision-based tests have more trouble than they expected—even if they think they’re sportin’ the best eyesight in town. Check out the tricky tests below. Do you have what it takes to pass them?

When it comes to eyesight, some people want you to know how good their vision is, while others brag about wearing glasses. That’s why people go crazy for these online eye tests. Sort of like, bragging rights for vision.

1. Okay, to kick us off, give this one a go. Stare at the Magic Eye Picture below. Relax your vision, like you’re staring into an open book you’ve already stopped reading. Give it a few seconds. Do you see anything?

Most people who stare down that Magic Eye Picture see three-dimensional penguins in the lower third of the picture—that’s because there’s some cool visual science at play here.

Magic Eye photographs started with the 1959 work of Bela Julesz. He tested peoples’ abilities to see in 3D with what he called a random dot stereogram. Then, in 1991, engineer Tom Baccei, 3D artist Cheri Smith, and programmer Bob Salitsky went further…

Rebecca / Flickr

They created images that looked 3D by putting darkening points that were “further away” and lightening the “close” points. Then, they overlay the two-dimensional pattern over it—the snow, in the case of the above photo.

When we look at the repeating pattern, our brain interprets the hidden, depth image—though it takes a few moments to parse through the weeds. But not everyone is so lucky.

Some people might look at the Magic Eye photos and never see the 3-dimensional figures hidden within them—they might as well have their eyes closed! Some people see the proverbial black and gold dress. But how?

To see a three-dimensional image, your eyes have to work together. They’ve got to be teammates. Partners. When that partnership fails, though, so does your stereo vision. In other words?

Out of sync eyes mean it’s tough for your brain to merge and blend the separate images they’re picking up (your eyes do view the world from slightly different positions, after all). No 3D vision for you. How about another one?

2. Are you ready to take an exam? All you have to do is spot the letter hidden in this picture in under seven seconds and you win. It’s not as easy as you think. Go ahead and give it a shot…

eye-test

Did you find it? It’s the letter “C,” tucked between the bottom right corner and center of the puzzle! See, not all eye exams are as easy as the others. Now, test your color vision with this tricky Picture…

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3. The below image, posted on Twitter by professor Akiyoshi Kitaoka, contains no red. Engineers scrubbed every trace of red from the pixels composing the photo—so why do so many people still see red strawberries?

Just as with the blue and black dress, viewers eyes process both the color of the object and the color of the light—and it often ignores the color of the light. Similarly, our brains know strawberries are redso they do some inferring.

“Your brain says, ‘the light source that I’m viewing these strawberries under has some blue component to it'” neuroscientist Bevil Conway, below, said. “‘So I’m going to subtract that automatically from every pixel.” The end result?

“When you take grey pixels and subtract out this blue bias, you end up with red,” he added. Undoubtedly, our eyes are unbelievable tools—even when we think our vision’s bad, they’re still working overtime!

4. That’s why, on February 26, 2015, Twitter users shared one highly-debated image over 10 million times in a single week. The picture raised serious questions about the hidden talents of our eyeballs—and drove the world into a social media frenzy.

See, some people who saw the overexposed, crummy photo (left) of the dress on the right saw a simple blue and black dress. Others looked at the exact same photo and observed a just-as-fashionable white and gold dress. How could this be?

Unbelievably, scientific communities never reached consensus as to how people saw two different dresses in the same photo. Neuroscientist Jay Neitz, right, offered a good theory, though.

Seattle Times / Ken Lambert
His jargon-heavy theory boiled down to this: we see the overexposed photo of a dress, and our brains eliminate what we perceive to be the color of light. Some brains see a white and gold dress lit with a blue light; others see a blue and black dress lit with gold light.

As Neitz put it, “Our visual system is supposed to throw away information about the illuminant and extract information about the actual reflectance.” So it depends what you saw as the color of the light. But what color was the dress really?

Blue and black according to Roman Originals, the dress’s retailer.  Lucky for them, the store saw a huge spike in sales after the photo went viral! It probably wasn’t intended as a marketing tactic, but it certainly worked out pretty well for them.

5. Now that you worked your eyes, see if you can solve this brain teaser? Upon first glance, this numerical nonsense may look like something you learned in second grade, but you won’t be able to solve it using traditional arithmetic…

1-impossible-math-equationhttp://junkhost.com
Are you stumped? You aren’t the only one! This challenge wouldn’t be nearly as interesting if the solution to it was straightforward. Luckily, there are two ways to solve this puzzle…

The first way to pinpoint the answer to this unusual question is to study all the numbers on the left side of the equation. What do they have in common, and how does that relate to the numbers on the right side?

It’s actually quite a bit less complicated to work out the solution to this problem than it may initially appear if you’re paying attention. For example, one plus five equals six, not 18. But six multiplied by three is 18. Could we have discovered the hidden code?

If you apply that same logic to the next set of numbers, you’ll notice it follows the sequence. Two plus 10 is 12, and if you multiply that by three as you did with the set of numbers above, you get 36. The same goes for three plus 15. So, what would four plus twenty, multiplied by three become?

The answer to the question mark is 72, of course! There’s one other method: if you look at the “answers” of each individual equation, you’ll notice that the scalar factor of the number nine gets raised by two each time: nine times two is 18, nine times four is 36, nine times six is 54. Therefore, nine times eight is 72!

5-impossible-math-equationhttp://giphy.com

6. Need a break from math, but still want to solve a puzzle? Try this riddle: as summer drew to a close, Prince Charming went out in search of his true love and came across a witch’s shack. The prince, exhausted by his quest, asked if he could stay with the witch for the night.

She obliged, but the following morning she presented him with a gift. “One day,” she predicted, “you will find your passage blocked by a wide river without a bridge to pass over it. Your only choice will be to swim to the other side. Fortunately, you’ll never be let down by this magic tunic!”

After thanking the witch, Prince Charming continued his journey, and 100 days later, he located the river that she’d told him about. Amazingly, however, he didn’t need the magical tunic to cross the river! Why do you think that was?

Remember: when Prince Charming met the witch, summer was almost over—meaning it was September. By the time 100 days had passed and he reached the river, the cold would have turned the water into ice, making it easy for him to simply walk to the other side!

7. For a different type of challenge, try this one: four men face the same direction while standing on descending steps, one in front of the other. However, a fourth man is separated from the group by a wall. He stands on the other side of it.

The first man can see the second and third men directly in front of him; the second man can only see the third man standing in front of him, and the third and fourth men can see no one.

Meanwhile, none of the men know what color hats they’re wearing on their own heads. All they know is that, among the four of them, there are two white hats and two black hats. They are instructed to shout out their hat colors once they’ve figured out which color it is.

The men can’t move or turn at all, they can’t talk, and their hats must stay on their heads. And, of course, the wall is separating the first three men from the fourth. With that in mind, you must answer: if they all follow the instructions, which one would be the first to shout by process of elimination, and why?

It’s the second man! He can confidently speak up because the third and fourth men can’t see the others, while the first man must know his hat is either black or white because he sees both colors on the men in front of him. So how would the second man know?

The second man knew because he only saw a black hat. Since he knew Man 1 behind him could see two hats, he should’ve shouted out his own hat’s color if Man 2 was also wearing black. But since Man 1 was silent, Man 2 could assume he wore a white hat. Hats off to you if you answered this correctly!

U.S National Archives / Wikimedia Commons

8. Here’s one last puzzle for you to exercise your brain: a man was changing his tire when all four of his lug nuts fell into a sewer grate. He had no way to retrieve them, and he wondered how long he’d have to wait on the street.

Fortunately, a girl riding by on her bicycle had a clever solution, and the driver had a way to put a new tire in and safely make his way to the closest gas station. What do you think the child explained to him?

The girl on her bike instructed the worried driver to temporarily take just one lug nut from all of the other tires on the vehicle, and use all of those to secure the replacement tire on the car.

From there, the man could drive reasonably safely for the time being until he could get it properly repaired. At that point in time, every tire had three lug nuts, which would be enough to keep the fourth tire in place!