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From the orange glow of a sunrise to the calming tones we choose to paint our bedrooms, color shapes the way we experience the world. It influences our moods, choices, and memories, and it adds a layer of emotional depth to everything we see.

Yet you may be surprised to learn that colors, despite being such an important part of the human experience, aren’t universally perceived the same way. Two people may look at the same object and experience the color differently, depending on various factors such as how their eyes and brains process light. Those variations remind us that the colors we take for granted are filtered through biology and culture, making our experience of the world more unique than we might realize.

How We Perceive Color

Color perception begins in the eye, specifically with the retina’s photoreceptor cells called cones. Humans typically have three types — S-cones, M-cones, and L-cones — that detect short, medium, and long wavelengths, roughly corresponding to blue, green, and red light. Those cones send signals to the brain, where they are interpreted as the colors we see.

However, not everyone’s eyes perceive color the same way. Color vision deficiencies, often called color-blindness, affect millions around the world — roughly one in 12 men and one in 200 women — shaping how they experience color. 

Red-green color-blindness makes it hard to pick out reds and greens, sometimes muting them or making them indistinguishable from each other. Less common forms include blue-yellow deficiencies, which blur blues, greens, yellows, and reds, and monochromacy (achromatopsia), which eliminates color entirely, causing people to see the world in shades of black, white, and gray.

Even subtle differences in cone distribution can alter perception. Two people gazing at the same sunset could describe it differently — one emphasizing the reds and oranges, the other noticing more purples and pinks.

Aging also plays a role: As the eye’s lens yellows over time, it begins to filter out blue light, lending colors a warmer, more muted tone. After cataract surgery, patients often report the world looking brighter or more vivid, a reminder of how physical changes in the eye affect what we perceive.

Color Perception Is Subjective and Changeable

Our brains’ interpretations of color aren’t fixed — they’re dynamic and easily influenced by context. One reason for this is a phenomenon called color constancy, which is the brain’s ability to recognize the same color under different lighting conditions. 

For example, a white shirt appears white in both sunlight and indoor lighting, even though the wavelengths reaching the eyes are very different. The brain adjusts for the light source, maintaining a consistent sense of color rather than shifting in hue.

Optical illusions, however, reveal the limits of color constancy. The viral internet phenomenon known as “the dress” — wherein some people saw a photo of a blue and black dress while others saw white and gold — shows how assumptions about lighting and shadow can completely alter our perception of color. 

In that case, viewers unconsciously “corrected” for what they thought the light source was, leading to dramatically different interpretations of the same image. This is the same principle filmmakers relied on in The Wizard of Oz.  The sudden burst of saturated color found in the Land of Oz feels especially vivid because our brains contrast it against the monochrome tones of the Kansas scenes.

Our everyday perceptions are just as personal. When choosing fruit at the grocery store, we use subtle color cues — such as the shift in hue from green to yellow in a banana or the deepening red of a ripe strawberry — to judge freshness. Some people focus more on saturation (how intense a color appears) rather than brightness (how light or dark the color seems), but both are part of how the brain interprets color in context. 

Even our physical and emotional states can change how we see color. Fatigue can dull our sensitivity to contrast, certain medications can change how light is processed by the retina, and mood has even been shown to influence perception: People with a positive mindset often perceive colors as slightly more vivid.

Our Understanding of Color Perception Is Limited

Even with advances in science, one puzzle remains: our inner experiences of color. The “inverted spectrum” thought experiment, which dates back to at least the 17th century and philosophers including John Locke, imagines a scenario in which one person’s red may appear to them as another person’s green, even though both call it “red.” 

This is a philosophical concept rather than a scientific claim, meant to illustrate the limits of our knowledge about subjective experience. Because we can’t step into another person’s perception, we can never know for certain how their colors compare to ours — or if two people truly experience the same hue the same way, even if they both call it “red.”

We know color perception involves a number of physical and psychological factors, but research is still uncovering just how personal and varied it can be. Genetics and neuroscience show that differences in our eyes and brain pathways shape our perception of the colors we see. Brain imaging studies reveal how the visual cortex interprets color, and experiments with color-blind individuals show how adaptable our brains can be to different ways of seeing.  

While we may never be able to fully view the world through someone else’s eyes, studying color perception can help us understand the fascinating mix of biology, culture, and consciousness that allows us to experience the world in our own unique ways.  

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Human beings have an extraordinary way of turning emotions into visible expressions. A burst of joy can light up the face with a smile, while a wave of sadness leaves our eyes brimming with tears. These physical reactions are caused by the remarkable relationship between the brain and body. 

When we experience emotions, a cascade of chemical signals and nerve impulses activates muscles and glands, producing observable expressions. Over millions of years of human evolution, these reactions have developed not only as individual responses but also as tools for social connection, signaling our inner state to those around us.

But why do certain physical reactions correspond to certain emotions? Why do the corners of our mouths lift up when we’re happy, and salty tears roll down our cheeks when we’re sad? The answers reveal a lot about the design of our nervous system, and the deeply social nature of human beings.

It All Starts With the Limbic System

At the heart of both smiling and crying lies the brain’s emotional control center: the limbic system. This network, which includes the amygdala, hippocampus, and hypothalamus, serves as a switchboard for our feelings. 

When we experience joy, the limbic system triggers activity in the brain’s reward circuits, releasing dopamine, serotonin, and endorphins. Those chemicals signal the muscles in the face, through cranial nerves, to contract in a way that forms a smile.

Sadness activates the same brain region, but in a different way. In this case, the amygdala and hypothalamus respond by activating the autonomic nervous system, which influences tear production. The lacrimal glands just above our eyes receive the signal and respond by producing emotional tears distinct from reflex tears (the kind that protect the eyes from dust or onions). 

In both scenarios, the limbic system is the unseen conductor, arranging expressions that not only reflect how we feel but also how we communicate it to the outside world.

Smiling Developed as a Social Tool

Smiling primarily involves the facial muscles, especially the zygomaticus major, which lifts the corners of the mouth. A genuine, involuntary smile — known as a Duchenne smile — also engages the orbicularis oculi muscles around the eyes, creating the crinkling effect that distinguishes true happiness from a polite grin. These smiles are triggered by the brain’s release of “feel-good” neurotransmitters including dopamine and serotonin, reinforcing pleasurable experiences.

Interestingly, the act of smiling can actually create a positive feedback loop in the brain. Psychologists have found when we smile, even if by force, it can trick the brain into releasing more dopamine and reinforce the sense of happiness. This is known as the facial feedback hypothesis, the idea that our expressions don’t just reflect emotions but can also help generate them.

From an evolutionary perspective, smiles likely developed as a social tool. A smile can signal friendliness, cooperation, or safety, helping people build social bonds. In early human societies, this would have strengthened trust and encouraged group cohesion — an essential advantage for survival.

How Crying Helps Us

Crying when we’re sad may feel like a loss of control, but it’s a deeply human response. Emotional tears differ from basal tears (also called continuous tears), which keep our eyes lubricated or the reflex tears that flush out irritants. Emotional tears contain higher levels of stress hormones such as cortisol, as well as endorphins that help stabilize your mood after crying.

This process begins in the hypothalamus, which detects emotional stress and sends signals to the parasympathetic nervous system. The lacrimal glands then produce tears, while the orbicularis oculi muscles contract around the eyes. Crying slows breathing and lowers heart rate, allowing the body to shift into a calmer state — a built-in form of self-soothing.

Like smiling, crying also has a strong social function. Tears are a nonverbal signal to others that we’re in need of comfort, support, or empathy, and studies show people are more likely to approach and console someone who is visibly crying. This social response likely explains why humans evolved the capacity for emotional tears — something no other animal seems to have.

Humans Evolved To Smile and Cry

Beyond individual biology, emotional expressions have deep evolutionary roots due to the social purposes they serve. Our friendly smiles and vulnerable tears promote group cohesion and mutual support to this day, but they were also vital for early human survival.

Research shows that emotional expressions are instinctive rather than learned. Babies smile and cry long before they can speak, suggesting these behaviors are hardwired. And cross-cultural studies confirm people around the world recognize basic emotions through facial expressions — even when factoring in cultural differences in how emotions are expressed — highlighting the universal biology at play here.

Throughout human history, evolution likely favored individuals who could effectively convey emotions to their community, ensuring understanding and cooperation within their groups.

Emotional Expressions Are Crucial

In modern life, our emotional expressions continue to serve important purposes. Smiling when we’re happy can strengthen our personal relationships, improve our mood, and even influence how others perceive us, while crying when we’re sad can provide relief and foster empathy. Brain imaging studies show observing a smile or tears can activate similar regions in the observer’s brain, demonstrating how expressions promote emotional connection.

In these ways, our face becomes more than just a reflection of your inner state — it also sends messages to others around us, reinforced by millions of years of evolution.

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Weather affects nearly every aspect of our daily lives  — from what we wear to how we travel and even how farmers plan their crops. Predicting the weather, however, is one of the most complex challenges in science, requiring large networks of satellites and sensors feeding information into powerful computers to mirror the dynamics of the ever-changing atmosphere. While it may seem like forecasting tomorrow’s rain or sunshine is a simple guess, it’s actually the result of careful analysis of data gathered from around the globe.

Modern weather forecasting blends centuries-old observation methods with cutting-edge technology. From satellites orbiting the Earth to sensors placed deep in the ocean, meteorologists track countless variables that influence the atmosphere. Those variables — including temperature, air pressure, wind, humidity, and more — interact in complex ways, creating the constantly shifting weather patterns that forecasters must interpret.

Farmer’s Almanacs

Long before scientific forecasting, people relied on folklore, observation of the skies, and seasonal almanacs to anticipate weather. The Old Farmer’s Almanac, first published in 1792, used a formula that was said to include solar activity, weather patterns, and astronomical cycles. 

Farmers and households turned to it for long-range forecasts, sometimes with uncanny accuracy, but just as often with results no better than chance. Folk sayings such as “red sky at night, sailor’s delight. Red sky in morning, sailor’s warning,” based on observations passed down through generations, also acted as early weather guides.

While those early methods provided some rough counsel, they lacked the precision we expect today. Weather forecasting as a true science began in the mid-19th century, when telegraph systems allowed rapid sharing of weather observations across regions, enabling the first warnings of approaching storms. Still, predictions in those days were only reliable a few hours in advance.

Observation

Today, the first step in predicting weather is gathering data. Meteorologists monitor the atmosphere using a combination of ground-based instruments, weather balloons, satellites, and radar. Ground stations measure temperature, humidity, wind speed, and air pressure in specific locations. Weather balloons carry instruments called radiosondes high into the atmosphere, providing readings of conditions at various altitudes. 

Each observation provides a piece of the larger puzzle. For example, a sudden drop in air pressure can indicate an approaching storm, while wind patterns help predict where weather systems will move next. Collecting this data continuously from hundreds of thousands of points on Earth allows meteorologists to track the ever-changing atmosphere with remarkable detail.

Modern Tools

Satellites and radar are essential tools for modern meteorology. Weather satellites were first launched in the 1960s, giving meteorologists a powerful new tool for weather observation. As they orbit the Earth, satellites provide continuous images and measurements regarding factors such as cloud cover, temperature, humidity, and wind at different altitudes. Some can even measure rainfall and detect severe storms including hurricanes or typhoons before they reach populated areas.

Radar systems complement satellites by offering detailed, real-time information about precipitation. Doppler radar in particular can measure the speed and direction of rain droplets, allowing meteorologists to detect rotation in storms, which is a key factor in predicting tornadoes. By combining satellite imagery, radar data, and surface observations, forecasters can track storms as they develop and issue timely warnings to protect lives and property.

Creating a Weather Model

Once data is collected, meteorologists turn to computer-based weather models to make sense of it. These models are mathematical simulations of the atmosphere that use physics and fluid dynamics to predict how weather systems will evolve. Essentially, the models take current conditions and calculate how they’re likely to change over time, producing forecasts that can range from a few hours to several days ahead.

There are many types of weather models, each with their own strengths. Some are global, simulating weather across the entire planet, while others are regional, focusing on a specific area for more precise predictions. 

Small changes in the atmosphere can have large ripple effects on the weather, so forecasters often run multiple models and compare results to find the most likely outcome. This process, called ensemble forecasting, helps account for uncertainty and increases the reliability of predictions.

Turning Data Into Forecasts

After collecting data and running models, meteorologists interpret the results to create forecasts for the public. They consider model predictions, historical patterns, and the effects of local geographic features, such as mountains or large bodies of water, which can influence local weather. While computers run billions of calculations, it’s human expertise that ensures the final forecast adjusts for uncertainties and translates complex data into useful information. 

Forecasts are usually presented as probabilities, such as a 70% chance of rain, to reflect the atmosphere’s unpredictability. Short-term forecasts can be highly accurate, sometimes down to the hour, while long-range forecasts beyond a week are less reliable. 

Still, accuracy has improved dramatically over the past few decades due to our evolving technology: A seven-day forecast today being as accurate as a three-day forecast was in the 1980s. According to the National Oceanic and Atmospheric Administration (NOAA), today’s five-day forecasts are accurate about 90% of the time, while seven-day forecasts are correct about 80% of the time. 

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In 1901, the Austrian immunologist and pathologist Karl Landsteiner made a breakthrough discovery: He found there were four major blood groups, which he classified in his ABO blood type system. Prior to his discovery, blood transfusions were incredibly risky and often fatal. Early attempts, including animal-to-human transfusions, frequently resulted in severe adverse reactions and were eventually banned across much of Europe. 

Some human-to-human transfusions were successful, but not consistently, and doctors struggled to understand why. Landsteiner’s work cracked the code, making safe blood transfusions possible for the first time in history — an achievement for which he received the Nobel Prize in 1930.

Today, blood typing saves millions of lives. But what exactly is the science behind it, and what do the different blood types mean? 

How Are the Eight Blood Types Different?

Our blood is made up of red blood cells, white blood cells, and platelets in a liquid called plasma. Our blood type, also known as our blood group, is determined by special proteins called antigens that sit on the surface of red blood cells. Landsteiner discovered that the nature of those antigens differs between people and that four main blood groups exist, which he defined as the ABO system. 

If you have type A blood, your red blood cells carry A antigens. Type B blood has B antigens and type AB has both A and B antigens. Type O blood, meanwhile, has neither A nor B antigens. But the classification doesn’t end there. 

In 1940, Landsteiner and his colleague A.S. Weiner discovered a second significant blood group factor based on the presence or absence of the Rh antigen, often called the Rh factor, on the cell membranes of red blood cells. Your blood type is classified as positive (+) if the Rh factor is present in your blood and negative (-)  if it’s absent. Taken together, this creates the eight common blood types: A positive, A negative, B positive, B negative, AB positive, AB negative, O positive, and O negative.

What Determines Your Blood Type?

Everyone inherits their blood type from their parents, just like eye color, height, handedness, and freckles. Blood types follow simple inheritance rules: Each parent gives you one blood type gene, and the combination determines your blood type. The genes for A and B blood types are codominant — in other words, they dominate equally — while the gene for blood type O is recessive. 

So if you inherit an A gene from one parent and an O gene from the other, you’ll have Type A blood because A is stronger. You need two O genes (one from each parent) to have Type O blood. If you receive both A and B genes, you’ll have Type AB blood since both are equally strong. 

Globally, the most common blood type is O positive, with more than a third of the population sharing it, followed by A positive. The rarest of the standard blood types are AB positive (2% of the population) and AB negative (1%), which can make finding a match difficult in some cases.

Rh status is also inherited from our parents, albeit separately from our blood type. If you inherit the dominant Rh antigen from one or both of your parents, then you’re Rh-positive (about 85% of the population). If you don’t inherit the Rh antigen from either parent, then you’re Rh-negative — and therefore your blood type will be negative. 

The Universal Donor

Blood type compatibility is crucial for enabling safe medical procedures. Before the discovery of blood types, blood transfusions that weren’t a match could result in clumping, or agglutination, of red blood cells. Those clumps could block small blood vessels throughout the body, depriving tissues of oxygen and nutrients, causing numerous problems and even fatality. 

Receiving blood from the wrong ABO group can be life-threatening because antigens present on red blood cells can trigger an immune response if they’re not compatible. For example, if someone with group B blood is given group A blood, their anti-A antibodies will attack the group A cells. 

The exception to this is type O, as those red blood cells don’t have any A or B antigens. Type O negative, specifically, can safely be given to any other blood type because it’s compatible with all groups. This is why people with O negative blood are considered “universal donors” (in the U.S., only about 7% of the population are O negative) and also why, during medical emergencies when the blood type is not immediately known, doctors will often use O negative blood. 

Conversely, people with O positive blood can only receive transfusions from O positive or O negative blood types, because their anti-A and anti-B antibodies would attack any donor blood with A or B antigens. The only blood type that can receive blood from any other type is AB positive, which is therefore known as the universal recipient.