Your body performs many functions that could be considered, well, unusual. Take yawning, for instance. It’s a mostly involuntary reflex brought on by various situations, such as being tired, bored, or even stressed. 

Though the mechanism isn’t fully understood, researchers believe yawning may help wake up your brain by increasing your heart rate, stretching your facial muscles, and reducing your brain’s temperature.

What’s even stranger, though, is that when you see someone yawn, it can trigger you to yawn, too. Even thinking about a yawn can cause one — you may have just yawned while reading this, in fact. Does that mean yawning is contagious? Colds are contagious, but you can’t catch one by reading about it. 

So what exactly makes a yawn — or even the thought of one — cause another yawn?

Copycat Brain Cells

Scientists have been trying to figure out why yawning is “contagious” for decades. One leading theory has to do with mirror neurons. These are specialized brain cells that were first discovered in the 1990s, which can fire when you perform an action as well as when you see someone else do the same thing. 

These neurons tell your brain to “mirror” the actions and emotions of others, and may help us learn from others. There’s still debate about the role mirror neurons play in yawning, but at least one functional magnetic resonance imaging (fMRI) study has shown that when you see someone yawn, the areas of the brain that control social cognition and imitation are activated. 

That means your brain subconsciously mimics the physical action of the yawn you just witnessed. It’s an automatic imitation response known as the “chameleon effect.” Mirror neurons may even be activated simply by reading or thinking about a yawn.

You can see an evolutionary advantage to the phenomenon. If you’re in a hot environment with someone and see them yawn to cool their brain temperature, you probably should be doing the same.

Humans Are Social Creatures

Other scientists, though, think contagious yawning has more to do with socialization and empathy. Social species, including lions, chimpanzees, and domestic dogs, might yawn together to help the group stay alert or shift from resting to waking states. 

Because of this highly social function, whether you mimic a yawn could be influenced by how close you are to the person doing the yawning. A landmark study found that you’re more likely to yawn when you see a relative or friend yawn than an acquaintance or stranger.

According to one 2010 study published by The Royal Society, contagious yawning also doesn’t begin at birth. In the study, it wasn’t observed in children until they were around age 4 or 5, when traits such as empathy — experiencing the emotions of others — become more developed. Not everyone is susceptible to contagious yawning. In studies where people were shown others yawning on video, between 30% and 60% responded by yawning back.

So why are some people less likely to catch yawns than others? Personality traits, empathy levels, intelligence, and genetics could all play a role. One small study found that people with high levels of psychopathic personality traits — including reduced empathy, mimicry, and group cohesion — are less susceptible to yawning contagiously. 

Even after decades of research, why we yawn when someone else does remains a challenge to answer with certainty. While empathy, social bonding, and copycat brain cells are all possible factors, maybe this strange phenomenon is just proof of how deeply connected we are as humans.

On a scorching summer day, having nothing but a thin layer of lotion standing between your skin and a sunburn can seem like the flimsiest of defenses. After all, it’s the sun, the most powerful source of energy in our neck of the galaxy. But somehow it works. 

How can something you paid just a few bucks for and squirt from a bottle protect you from powerful UV radiation blasting your skin for potentially hours at a time?

Sunscreen isn’t a form of medicine that fights skin damage like your immune system might. It’s more like a bouncer, stopping most of that solar radiation at the door before it can ruin your skin’s party (or at least, your afternoon at the beach).

Catching Rays

A sunburn isn’t a heat burn, like the kind you might get from grabbing a hot pan handle. Instead, it’s your body’s own response to damage caused by ultraviolet (UV) radiation that, while invisible, makes up about 5% of sunlight reaching the surface. Think of the visible wavelengths of light as the colors of the rainbow: red, orange, yellow, green, blue, indigo, and — finally — violet. UV is what comes next, hence “ultraviolet.” 

When UV rays penetrate the skin, they can physically alter cells, damaging them and your DNA. In response, the body launches an inflammatory reaction, increasing blood flow to the area and causing redness, tenderness, and swelling.

Sunscreen helps prevent sun damage in the first place by forming a protective film on the surface of the skin. This film contains special ingredients that serve as UV filters, like microscopic pairs of sunglasses for your skin cells. The filters interact with incoming ultraviolet radiation before it can penetrate deeply enough to wreak havoc.

Sunscreens are often divided into two categories: chemical and mineral. Chemical sunscreens contain organic UV filters — not “organic” in the grocery store sense, but meaning they contain carbon-based compounds (which can be unnecessarily absorbed into your bloodstream). Mineral (or physical) sunscreens contain inorganic ingredients, usually zinc oxide, titanium dioxide, or both. Many modern sunscreens combine ingredients from both categories.

For years, people were often told that chemical sunscreens absorb UV rays while mineral sunscreens reflect them. Scientists now know the story is more complicated. Most sunscreen ingredients, including the mineral filters zinc oxide and titanium dioxide, protect the skin primarily via the same process. And that process is absorbing UV radiation.

When a UV ray strikes one of these ingredients in your sunscreen, its energy is absorbed and converted into a tiny amount of heat. The amount of heat is so small that you can’t feel it, but it means the UV energy is no longer available to harm your skin. In a sense, sunscreen gives UV energy somewhere else to go before it can collide with your DNA.

The Myth of SPF

Not all UV rays are the same. UVB rays are the chief sunburn culprits, while UVA rays penetrate more deeply and contribute to wrinkles and other signs of skin aging. That’s why dermatologists recommend broad-spectrum sunscreens, which are designed to protect against both.

The SPF on your sunscreen bottle stands for “sun protection factor.” Many people think SPF measures how long you can stay in the sun without burning, but that’s a common misconception. Instead, it tells you how much UV radiation you can absorb before starting to sunburn. 

It’s an important distinction, because the amount of UV radiation you receive at any given moment depends on many factors, including the time of day, cloud cover, your skin type, and how much sunscreen you apply. Never assume you’re safe from burning for 30 minutes simply because you’re using SPF 30.

In general, a higher SPF provides greater protection against UVB rays, the main cause of sunburn. The Skin Cancer Foundation recommends wearing SPF 50+ when you’re outdoors for an extended time and SPF 30+ for typical daily use. It also recommends using a water-resistant broad-spectrum sunscreen. That’s because sunscreen isn’t some magical, impenetrable force field. 

Water (from sweat or the pool) will break down its formula, as will friction. Plus, people tend not to apply the recommended amount. Eventually, it will wear off, maybe sooner than you need, and some UV radiation will still reach your skin. 

That’s why dermatologists recommend combining sunscreen with other forms of protection, such as hats, sunglasses, protective clothing, and good old-fashioned shade. (Trees have been in the sunscreen business for a very long time.)

You know the drill: You wake up in the middle of the night, stumble toward the bathroom, and wait for your eyes to adjust. It feels like it takes a few seconds, but in reality, your night vision doesn’t hit its peak until about 30 minutes after the lights go out. Even then, one glance at your blindingly bright phone resets the whole process. 

That’s because your eyes adjusting to low light isn’t a mechanical process, and you’re not just “getting used to” the dark. Instead, there’s a specific chemical your eyes spend all day using up — and all night trying to rebuild.

In Rods We Trust

Light enters your eye through the pupil, the dark “hole” in the middle. Located in the back of your eyeball, the retina is the part of your eye that converts the light into the images we see. The retina has a light-sensitive layer lined with two types of photoreceptor cells: cones, which handle color and detail in bright light, and rods, which take over in the dark. 

A whopping 95% of photoreceptors are rods, which are far more sensitive to light than cones. They can’t see color at all, though, and aren’t great with details, which is why our night vision is never as good as our day vision). Rods rely on a protein called rhodopsin to do their magic, while cones use one called photopsins.

Here’s the problem: Both of these proteins break down when they’re exposed to light, in a process known as photobleaching. You’ve likely experienced concentrated bouts of photobleaching — for instance, after a day at the beach. Step inside after hours of practically staring at the sun and even an adequately lit room will seem dim and hard to see.

That’s because you’ve essentially overfed your light-sensitive rods, making those cells essentially offline until your eyes have a chance to rebuild their supply of rhodopsin. Even if you’re not at the beach, the same thing happens throughout the day, only more gradually.

The Speed of Dark

Your eyes get to work adjusting as soon as you step into the dark. First, muscles in your eye dilate, or widen, your pupils to let in as much light as possible. Your cones kick into gear first, reaching their peak sensitivity (which isn’t very much compared to rods) within about five to ten minutes. 

At this point, the rods — the ultrasensitive cells that actually make night vision possible — are still warming up. They need about 20 to 30 minutes to fully recharge. Until then, you’re working with a limited night vision. Once your rods are fully recharged, the difference is dramatic. 

Think about when you first walk outside at night and look at the sky (assuming you’re not in a bright city). At first, it just looks like a flat, black ceiling. But as time goes by, you’ll start to see stars and other details. That’s because after about 30 minutes outside, your eyes have become around 10,000 times more sensitive to light than they were when you first stepped outside.

Now you can see why waiting a half hour for your rods to get ready to work is worth it. But the downside is that all of that progress can be wiped out in seconds. A single bright light — including a glimpse at your cell phone screen, which is usually set to a brightness meant for daytime — breaks down the rhodopsin your rods just spent all that time rebuilding. 

Your night-vision goes down the drain and there’s no way to speed up the recovery. You just have to wait — or turn on a lamp.

Paper cuts can strike from anywhere — while loading paper into your printer, peeling off a sticky note, or turning the page of your favorite book. Afterward, you can barely even see where this particular blend of cellulose fibers sliced into your skin, but you know it’s there because it hurts — a lot.

How can such a tiny slash feel so painful? The answer actually comes down to a mix of factors that come together to form a perfect storm of agonizing discomfort. Warning: It may be tough to read on without wincing once or twice.

Paper-Thin Razors

While paper is not typically seen as a sharp and dangerous weapon, its edge tells a different story when seen under a microscope, sporting sharp, jagged teeth. Though tiny, these razor-like points are why paper is more hazardous than it seems on, well, paper.

A straight-edged knife makes a clean cut, causing less overall damage to the skin than a serrated one. In effect, you’re being cut many times instead of once when you make contact with the edge of a piece of paper, with the multiple jagged points hooking into your finger and tearing at it as it runs alongside the paper.

That said, not all types of paper are equally dangerous. Scientists have calculated that paper cuts are most likely to occur from sheets with a thickness near 65 microns at an angle of around 20 degrees from the skin. Note cards, for example, are (usually) too thick to cut into the skin, while material like toilet paper is too thin and soft to hold an edge firm enough to injure you. The paper used in standard print magazines, though, are closer to 65 microns thick, so flip through one at your own risk.

Paper cuts also may sting more than larger cuts because paper can leave behind tiny fibers, as well as traces of chemicals used in paper manufacturing, that further irritate the open wound.

Bad Feelings

But the mechanics and composition of paper are only part of the problem. Because of the way we usually handle paper, paper cuts typically occur around sensitive areas densely packed with sensory neurons known as nociceptors — which specifically detect damage and pain. Researchers suggest that in early adolescence, the brain develops an oversensitivity in our fingers, meaning we essentially feel more in those areas. 

Our brain’s somatosensory cortex, which translates touch perceptions from our hands into signals the brain can interpret, is stimulated more when there’s a greater density of sensory nerves. You’re much more sensitive to touch or pain on your lips, which have many such nerves, than the middle of your back, where they are more spread out.

It makes sense our fingertips are loaded with these sensory neurons, since they’re a gateway to our sense of touch. They’re what we use to carefully grasp objects and feel subtle details on a surface.

But, this heightened sensitivity is a double-edged sword (or paper envelope). While running our fingertips across velvet feels especially satisfying thanks to fingers’ enhanced touch receptors, running them over the edge of paper is just the opposite, especially since they can sense its near-invisible sharp points. 

So when we feel a paper cut on our finger, we really feel it.