Why Is Pee Yellow?

Chloe Effron
Chloe Effron

WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your body is kind of like a house. You bring things into your body by eating, drinking, and breathing. But just like the things we bring home to real houses, we don’t need every part of what we take in. So there are leftovers, or garbage. And if you let garbage sit around in your house or your body for too long, it gets gross and can make you sick. Your body takes out the garbage by peeing and pooping. These two things are part of your body’s excretory system (ECKS-krih-tore-eee SISS-tem), which is just a fancy way of saying “trash removal.” If your body is healthy, when you look in the toilet you should see brown poop and yellow pee.

Clear, light yellow pee is a sign that your excretory system and the rest of your body are working right. If your pee, or urine (YER-inn), is not see-through, that might mean you are sick. Dark yellow urine usually means that you aren’t drinking enough water. On the other hand, really pale or colorless pee can mean you might be drinking too much water! 

Your blood is filtered through two small organs called kidneys (KID-knees). Remember the garbage we talked about earlier? The chemicals called toxins (TOCK-sins) are like garbage in your blood. Your kidneys act like a net, catching the toxins and other leftovers and turning them into pee.

One part of your blood is called hemoglobin (HEE-moh-gloh-bin). This is what makes your blood red. Hemoglobin goes through a lot of changes as it passes through your body. When it reaches your kidneys, it turns yellow thanks to a chemical called urobilin (yer-ah-BY-lin). Urobilin is kind of like food coloring. The more water you add, the lighter it will be. That's why, if you see dark yellow pee in the toilet, it's time to ask your mom or dad for a cup of water. 

To learn more about pee, check out this article from Kids Health. 

11 Insightful Facts About Eyes

iStock.com/Paffy69
iStock.com/Paffy69

There are a lot of myths and misconceptions about the eyes. No, sitting too close to the TV won't damage your vision, and reading in dim light won’t hurt either. It’s understandable that various parts of the eye are so little understood, though. Each eye has more than a million optic nerve cells and over 106 million photoreceptor cells, making it one of the most complex organs we have. Here are a few more things you should know about your “windows to the soul.”

1. Newborn babies see the world in black and white—and red.

“It is a myth that babies see in black and white,” Anna Franklin, leader of the University of Sussex's Baby Lab, told The Guardian. While newborns do see black, white, and shades of gray, they can also detect red objects against a gray backdrop, Franklin says. The reason why they can’t see more colors is because the cones in their eyes—the photoreceptor cells responsible for picking up colors—are too weak to detect them. Those cells quickly get stronger, though. After about two months, babies can distinguish between red and green, and a few weeks later they can tell the difference between blue and yellow.

2. Your eyeballs grow as you age.

Another common misconception is that your eyes remain the same size from birth to adulthood. As a newborn, your eyes measure about three-fifths of an inch from front to back, compared to a little under an inch in adults. Your eyes actually grow a great deal in the first two years of life, and another growth spurt occurs when you go through puberty. The confusion likely stems from the fact that your eyes as a 6-month-old infant are two-thirds the size they will be when you’re an adult.

3. The length of your eye partly determines how well you'll be able to see.

If your eyeball is too long or too short, you might end up having problems with your vision. Nearsighted people have eyes that are longer than average, while farsighted people have eyes that come up a little short. If you were to magically add or remove a millimeter of length from your eye, it would completely change your prescription. Aside from eye length, the shape of your cornea (the outer part of the eye where contact lenses are placed) and lens (the part of the eye located behind the iris and pupil) are other key factors that determine the quality of your vision. That's because both of these parts work together to refract light.

4. Contact lenses can't really get lost behind your eye.

Although it may feel like a dislodged contact lens is stuck behind your eye, that isn’t exactly what’s happening. The thin membrane covering the white part of your eye and the underside of your eyelid—called the conjunctiva—forms a pouch and prevents objects from getting behind your eyeball. If a contact lens gets shifted out of place to the point where you can no longer see it, it’s just stuck underneath your upper eyelid, which isn’t nearly as scary.

5. Blue-eyed people share a common ancestor.

Originally, everyone in the world had brown eyes. It wasn’t until around 6000 to 10,000 years ago that the first blue-eyed person was born as a result of a genetic mutation, according to a 2008 study. That mutation of the OCA2 gene essentially “turned off the ability to produce brown eyes” and diluted the color to blue, Professor Hans Eiberg of the University of Copenhagen said in a statement.

6. Parts of the eye can get sunburned.

There’s a good reason you should wear sunglasses when it’s bright outside. Too much exposure to UV rays can damage the surface of the cornea and conjunctiva, causing a condition akin to sunburn called photokeratitis. Symptoms include pain, red or swollen eyes, the sensation of a foreign body in the eyes, blurred vision, headaches, and seeing halos around lights. While the discomfort is temporary and tends to go away within 48 hours, longer exposure to UV rays can have a long-term effect on your vision and lead to macular degeneration (deterioration of the retina, which is often age-related) and cataracts (clouding of the eye's lens, which reduces the amount of light coming in).

7. Your eye muscles are the fastest muscle in your body.

Extraocular muscles are what let you look around in all directions. You have six of these muscles in each eye, and many of the motions they make are involuntary. This lets you flick your eyes to one side and notice something in your peripheral vision without consciously looking in that direction. When both of your eyes move in the same direction, the movement is called a saccade, which comes from the French word for “jerk” (the verb, not the person). These jerky movements are extremely rapid, lasting about 50 to 60 milliseconds per saccade, according to Dr. Reza Shadmehr, professor of biomedical engineering and neuroscience at Johns Hopkins University. “Saccadic eye movements are the fastest voluntary movements that we can make. The eyes move at around 500 degrees per second or more,” Shadmehr tells Mental Floss.

8. Your eye movements might give away your next move.

Shadmehr and other researchers conducted an experiment in 2015 to test the relationship between saccades and decision-making. Participants were placed in front of a computer and asked to choose between two options that appeared on the screen: an immediate reward and a delayed reward. For instance, one option might be “get $10 today,” while the other might be “wait 30 days and get $30.” Their eye movements were tracked the entire time, and researchers discovered that these movements gave away the choice they were about to make before they made it. At the last minute, their eyes would move at a faster velocity towards the option that they preferred.

“What’s interesting is that as the saccades are being made, the velocity of the eyes starts out being equal between these two stimuli, but then right before you decide ‘I like A better than B,' the saccade that you make toward A has a higher velocity than the one you make toward B,” Shadmehr explains. “The idea is that the way you’re evaluating things is reflected in the way you move toward them.”

In another experiment, Shadmehr found a correlation between faster eye movements and impatient and impulsive behaviors. Similarly, other studies have shown that our eye movements are linked to moral decisions and even our political temperament.

9. You can tell some animals' place in the food chain by looking at a part of their eye.

In 2015, vision scientist Martin Banks and his colleagues looked at the eyes of 214 species in an attempt to answer the question, “Why do animal eyes have pupils of different shapes?” By the end of their study, they noticed a few patterns. Predatory animals like big cats and snakes tend to have pupils in the shape of vertical slits. This particular shape gives them the advantage of being able to accurately judge the distance separating them and their prey, so they'll know exactly how far they have to pounce. On the other hand, horizontal pupils are more common in goats, deer, cattle, and other herbivores. This shape improves an animal’s panoramic vision, which helps them look out for predators.

10. An eye condition may have been partly responsible for Leonardo da Vinci's artistic genius.

Visual neuroscientist Christopher Tyler argued in a recent paper that the master artist behind Mona Lisa had strabismus, a disorder where the eyes are misaligned. Essentially, one of his eyes turned outwards, and he was able to use both of his eyes separately (monocular as opposed to binocular vision). Tyler believes this actually aided his art by improving his ability to render three-dimensional images on a flat canvas. “The condition is rather convenient for a painter, since viewing the world with one eye allows direct comparison with the flat image being drawn or painted,” Tyler said. We’ll never know for sure whether or not this was true for Leonardo, but it’s an intriguing theory.

11. SURGEONS HOPE TO BE PERFORMING WHOLE EYE TRANSPLANTS BY 2026. 

Currently, only cornea transplants to improve vision are possible, but a team of Pittsburgh-based transplant surgeons said in 2016 that they hoped to be performing whole eye transplants in humans within the next decade. Transferring an eye from a deceased donor to a recipient certainly won’t be easy, though. A complicated network of muscles, blood vessels, and nerves connects the eyes to the brain via the optic nerve. However, further studies into the optic nerve and recent advances in immunosuppressive drugs and surgical techniques have brought them several steps closer to achieving this goal. If successful, the surgery could restore vision to people who have suffered severe eye injuries. Their research is backed by the Department of Defense, which is concerned about the number of soldiers who sustain eye injuries in combat.

11 Facts About Eczema

iStock.com/Anetlanda
iStock.com/Anetlanda

If you’ve ever had to deal with dry winter skin, you may think you know what eczema feels like. But anyone living with the chronic condition will tell you it’s much more than that. Rashes can rear their heads at any time of year, and eczema causes include irritants as mundane as food, clothes, and the weather. Symptoms range from mildly annoying to distracting enough to keep people from getting sleep and focusing on their work. Here are some more facts about eczema causes, symptoms, and treatments.

1. Eczema isn't just one condition.

Rather than describing one specific skin condition, eczema is used as a catch-all term for a group of related conditions. When people mention eczema, they’re often referring to atopic dermatitis: This is a chronic inflammatory condition characterized by dry, itchy red patches that flare up across the body when the immune system overreacts to a trigger. There’s also contact dermatitis, which is when rashes are triggered by irritants coming in contact with the skin; nummular eczema, when rashes are coin-shaped; and stasis dermatitis, when fluid “weeps” out of weakened blood vessels in the skin. Eczema shouldn't be confused with psoriasis: While both are conditions that lead to dry, itchy skin, the latter is an autoimmune condition while the former is primarily caused by allergic reactions.

2. Eczema is sometimes limited to the hands.

Eczema doesn’t have to affect the entire body. Hand eczema comes with many of the same symptoms of regular dermatitis—including chapped skin, painful cracks, red patches, and itchy blisters—but is limited to the hands and forearms. Many people without eczema deal with dry hands, especially during the colder months, so it can be difficult to know when these symptoms are signs of a medical condition. If your itchy, irritated hands can’t be treated with moisturizer alone, ask a dermatologist if you may have hand eczema.

3. It's often genetic.

Your genetic background is a strong predictor of eczema. If both of your parents have it, there’s an 80 percent chance that you will develop it as well, according to the Eczema Association Australasia. A family history of asthma and hay fever is also linked to eczema.

4. Eczema can be debilitating.

A list of symptoms doesn’t begin to capture what the experience of living with eczema is like. The itching and discomfort that comes with it can be so intense that it keeps people up at night, leaving them exhausted and unable to function during the day. Symptoms may be so acute that they’re all the person thinks about, which can hinder their relationships and work life. Others may feel discouraged to go out in public because they’re self-conscious of how their skin looks.

5. It's not contagious.

No matter how much contact you have with someone with eczema, there’s no chance of you catching their skin condition. Despite this, many people will see someone scratching their eczema rashes and assume what they have is just as contagious as poison ivy or chicken pox. And because eczema often runs in families, it sometimes carries the illusion of “spreading” between people who live together.

6. Eczema can be triggered by your environment ...

There are a number of environmental factors that can trigger eczema symptoms. Changes in climate—either to cold, dry conditions or hot, humid ones—may be enough to provoke a flare-up. For many people, chemical irritants are the sources of their rashes. Cigarette smoke, perfumes, household cleaners, shampoos, and fabrics like wool and polyester have all been linked to eczema reactions. That doesn't mean that everyone with eczema needs to avoid these things: The condition affects everyone differently, and an irritant that causes one person’s flare-ups may have zero effect on someone else.

7. ... and stress level.

Even if someone with eczema takes great pains to avoid their environmental triggers, a hard day may be all it takes to make their skin break out. Many people with eczema report exacerbated symptoms when they're feeling stressed. According to the National Eczema Association, eczema sufferers are more likely to be diagnosed with anxiety and depression, creating a vicious cycle for patients who count stress as a trigger.

8. It's connected to allergies.

Eczema is often accompanied by an allergic condition, whether it’s asthma, hay fever, or allergies to food. Up to 80 percent of children with atopic dermatitis go on to develop asthma or hay fever. It’s unclear what the exact relationship between allergies and eczema is, but some medical experts believe that the weakened skin barrier associated with eczema makes it easier for allergens to enter the body, which can in turn impact the immune system over time. Allergens—like pollen, dust, pet dander, and mold— are also triggers for some people with eczema.

9. Scratching makes it worse.

When an eczema flare-up starts to itch, it can be impossible to think about anything else. But scratching a rash is the last thing people with this condition should do. Instead of relieving discomfort, scratching a dry patch of skin can irritate it even further. Sometimes eczema sufferers scratch their skin so much that it starts to bleed, opening the door for potential infections.

10. Eczema is more common in kids.

Eczema affects roughly 11 percent of U.S. children [PDF] and 7 percent of adults [PDF]. Most kids with eczema develop it within the first five years of life, with 65 percent percent of child eczema patients first showing symptoms as infants. Living with eczema can be taxing for both kids and parents—especially when kids can’t stop themselves from scratching their rashes—but fortunately, half of kids with the condition grow out of it by the time they reach their teen years.

11. It can't be cured—but it can be treated.

There’s no cure for eczema, but there are some treatments that can help keep aggressive symptoms under control. Above all, eczema patients should keep their skin clean and moisturized to prevent flare-ups. Doctors recommend taking regular showers with warm water (but not hot water, as that can dry skin even more), and applying moisturizer immediately after bathing. If regular moisturizer isn't enough to soothe skin, doctors may prescribe a topical ointment with steroids to reduce inflammation, and if that still isn't effective, systemic medications that fight inflammation throughout the whole body may help. Ultraviolet B therapy is another treatment option. A few times a week, patients stand in a UVB light box that mimics natural sunlight. This encourages vitamin D production and curbs skin's inflammatory response while calming itchiness at the same time.

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