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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.   

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The digestive system is more than a collection of organs that turn food into fuel: It’s an elaborate, highly coordinated process that starts the moment you chew something and doesn’t finish until nutrients are absorbed and waste is cleared. 

From a rumbling stomach to the trillions of microbes living in your intestines, digestion affects everything from your mood to your concentration and overall health. Most of us likely only notice it when we’re hungry (or overstuffed), but there’s a lot going on under the surface. Here are seven surprising facts about the digestive system.

The Brain and Belly Are in Constant Conversation

The gut isn’t called the “second brain” for nothing. Trillions of microbes live in the digestive tract, breaking down food, producing vitamins K and B, and supporting nutrient absorption. They communicate with the brain through the gut-brain axis, a network of nerves (such as the vagus nerve) and chemical messengers, including serotonin and its receptors in the gut. 

This bidirectional communication helps coordinate digestive activity, appetite, and even certain aspects of mood and behavior, which explains why your stomach may “flutter” when you’re nervous or why certain foods can affect how you feel. While the brain doesn’t control every gut movement, this system helps the digestive tract respond dynamically to both internal and external cues.

Stomachs Don’t Just Growl When You’re Hungry

Everyone has occasional hunger pangs, but to actually hear and feel your stomach growl is a whole other sensation. The same muscle contractions that move food through your gut while you’re digesting are also what cause that gurgling sound and feeling.

When your stomach is empty, your brain releases the appetite-stimulating hormone ghrelin. This tells the digestive muscles in the stomach and intestines to contract in waves, a process called peristalsis. Those contractions create the vibrations and rumblings you hear, made louder by the fact that the empty stomach means there’s nothing to muffle the sounds.

Contrary to popular belief, the noise doesn’t always signal feeding time; even a full stomach can growl as it churns and clears leftovers for the next meal. The rumbling sound — technically called borborygmi — comes from the Greek “borboryzein,” meaning “to rumble.” Unless it’s accompanied by unusual pain or bloating, borborygmi is a normal, healthy sign your digestion is working as it should.

Stomachs Can Hold as Much as a 2-Liter Soda Bottle

This probably won’t come as a surprise if you’ve ever felt your pants get a bit more snug after a big meal, but the stomach is remarkably elastic and can expand to hold a lot of food. This is because its inner surface, the mucosa, is lined with folds called rugae that unfold and stretch out to accommodate food as it arrives. 

Adult stomachs start out folded up and roughly the size of a 16-ounce soda can. From there, the stomach can comfortably enlarge to hold about 1 to 2 liters of food and drink. If you picture that amount of sustenance as a 2-liter soda bottle, it sure doesn’t sound comfortable, but thankfully the stomach is able to slowly retract after digestion.

We Swallow a Lot of Air 

There isn’t much you can do to avoid swallowing air — every bite or sip brings tiny pockets of it into your digestive system as you eat. Overall, the average person swallows up to 2 quarts of air every day. Some small amounts can actually aid digestion, but habits such as eating quickly, chewing gum, or drinking carbonated beverages make for a greater volume of swallowed air. So where does it all go? 

Most of the air makes its way to your stomach and leaves the body naturally through burping or flatulence. While most rumbles and bubbles are proof your gut’s digestive train is on track, too much air in the digestive system — called aerophagia — can cause bloating, discomfort, and excess gas. 

Intestines Are Impressively Long

The digestive system starts at the mouth and ends where waste exits the body, but the vast majority of it is made up of the intestines. The small intestine alone can be as long as 25 feet, and the large intestine adds approximately another 5 feet. Stretched out straight, that’s around the same length as an average school bus — just all coiled up and tucked out of sight inside your abdomen. 

The small intestine does most of the work when it comes to breaking down food and absorbing nutrients, using its folds and tiny finger-like projections, called villi, to capture as much fuel for your body as possible.The large intestine then absorbs water and minerals while converting what’s left into solid waste. The intestines, particularly the small one, are workhorses, constantly moving food along, mixing it with digestive juices, and making sure your body gets everything it needs from what you’ve consumed.

The Stomach Lining Is Always Renewing Itself

Digestive acids are powerful — so powerful that if your stomach didn’t protect itself, it could start dissolving its own tissue. To prevent this, the stomach lining, called the epithelium, is in a continuous state of renewal. And the turnover is pretty rapid: Cells regenerate every three to four days, ensuring the stomach can withstand the corrosive effects of the 3 to 4 liters of hydrochloric acid and digestive enzymes it produces every day. 

Even with the constant regeneration, the acids that break down proteins and aid nutrient absorption can be strong enough to start eroding the stomach tissue. Some factors can exacerbate this, such as excessive alcohol or pain reliever use, certain bacterial infections, and chronic stress.

We’re Not Born With the Healthy Bacteria Needed for Digestion

Babies enter the world free of worry and responsibility — and also free of a fully formed digestive system. Newborns are largely devoid of bacteria and other microbes essential for digestion and health. Their gut microbiomes first begin forming as they pick up bacteria in the birth canal, then continue to grow and diversify through exposure to breast milk, formula, and their environment. (Babies born by caesarean section get their first dose of bacteria in other ways, primarily from skin-to-skin contact and feeding.)

By the time a child is about 3 years old, the microbiome has become more diverse and stable, playing a key role in lifelong digestive health. This process highlights just how dependent humans are on our microbial communities: Digestion isn’t purely mechanical, but rather a cooperative effort between your body and the trillions of tiny organisms that call your gut home.

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Perfect pitch (or absolute pitch, as it’s formally known) is the rare ability to identify or reproduce a musical note without any external reference. Imagine hearing a single note on a piano and instantly knowing it’s a C-sharp, or whistling “Happy Birthday” in the exact intended key without a musical instrument or pitch pipe nearby. It’s a skill that fascinates musicians and nonmusicians alike, partly because it seems almost magical, and partly because fewer than one in 10,000 people possess this rare gift. 

For centuries, musicians and theorists have debated whether perfect pitch is a natural gift, a learned skill, or a mix of both. Historical accounts, including letters from classical composers, suggest some musicians were identified as child prodigies precisely because of their uncanny ability to instantly recognize notes. 

Mozart, Beethoven, Chopin, and Bach are often cited as having had perfect pitch, although documentation about their pitch abilities is mostly anecdotal. And perfect pitch does appear to have a genetic component: Studies show identical twins are more likely to share absolute pitch abilities than fraternal twins.

So what exactly causes perfect pitch? And just how prevalent is it? Let’s take a closer look at this unusual and fascinating ability. 

It’s a Combination of Nature and Nurture

Despite the genetic link, perfect pitch isn’t just a naturally inherited trait. Early musical training plays an important role in honing the skill, especially for children exposed to musical instruction before age 6. Research suggests the brain is particularly receptive to auditory patterns during this period, allowing young learners to encode pitch information in a way that sticks for life. 

Children who start music lessons after 6 rarely develop perfect pitch, even if they’re highly talented. This is one reason why perfect pitch is so uncommon — most people don’t receive the necessary musical training at such an early age.

Interestingly, perfect pitch is not evenly distributed across cultures or languages. Studies have found higher prevalence in speakers of tonal languages, such as Mandarin or Vietnamese, compared to speakers of nontonal languages such as English. 

In tonal languages, subtle pitch differences can change the meaning of a word, which means the brain may become more attuned to auditory distinctions early in life. This suggests perfect pitch may not be entirely about music — it may also reflect a heightened sensitivity to sound that’s supported by both language and environment.

It Starts in the Brain

Neurologically, perfect pitch is associated with distinct brain structures. Imaging studies have found differences in the auditory cortex of people with absolute pitch, particularly in areas responsible for processing tone and pitch memory. The brains of people with absolute pitch often show stronger connections between the auditory and memory regions, which may help people better retain and recall precise pitch information. 

However, perfect pitch doesn’t necessarily mean better musicianship overall. Many skilled musicians have relative pitch — a similar but more flexible ability to recognize intervals and harmonies — but not absolute pitch. In fact, some experts argue relative pitch is more valuable for composition, improvisation, and ensemble performance because it relies on context rather than memorization of individual notes.

Perfect pitch also has its quirks. Some musicians with absolute pitch report challenges in certain situations, such as transposing music or adapting to instruments tuned differently from the standard pitch. Their brains are so finely attuned to exact frequencies that even minor deviations can throw them off.

It Requires the Perfect Combination of Factors

So why do only some people have perfect pitch? Genetics, early musical exposure, language environment, and brain wiring all play roles, but the interaction between those factors is complex. Some researchers believe a person must inherit a predisposition for heightened pitch sensitivity and then receive early, structured musical training to fully develop absolute pitch. Without either component, perfect pitch may never emerge.

Because it’s so rare, people often imagine perfect pitch as a sign of genius that sets certain musicians apart. While it can certainly enhance some aspects of musical performance, it’s not a prerequisite for musical success. In fact, some of the greatest composers in history — including Leonard Bernstein, Tchaikovsky, and Wagner — didn’t have it.

There’s also debate about whether perfect pitch can be learned later in life. Some adult learners claim to have developed absolute pitch through intensive practice or software-based training. 

And while there’s some evidence that adults can improve their pitch identification skills, most studies indicate that accurate, effortless perfect pitch is extremely difficult to acquire after childhood. The brain’s auditory pathways become less malleable over time, meaning later attempts to train the brain through concerted effort rarely reach the level of natural absolute pitch.

So yes, perfect pitch is a rare but very real trait. Beyond the fascination it inspires as a musical curiosity, it offers an intriguing look at the intersection of biology, culture, and human potential, reminding us that the way we hear the world can be as unique as the music we create within it.