The human body is a story of resilience. Wounds heal, bones regenerate, and our immune system fights diseases and infections, yet few parts of the body experience as many challenges as our teeth. Capable of biting, tearing, and grinding, teeth are designed to withstand (almost) anything humans consume, and evolution has provided Homo sapiens some help in the form of tooth enamel. Formed from a mineral known as calcium phosphate and arranged in a crystal lattice “woven” with threads 50 nanometers across (1,000 times smaller than a human hair), enamel is the hardest substance in the human body. Its lifelong mission is to protect the innermost layers of the tooth, including the dentin and tooth pulp (which contains all those nerves that give you a toothache). 

Some people are born without wisdom teeth.

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Between 5% and 37% of people are born missing one or more wisdom teeth. A study in 2020 concluded that more babies are being born without these third molars, which could be an example of microevolution as humans lose the need for them.

A human’s extra-strength set of chompers is useful for eating everything from warm soup to rock candy, but teeth aren’t invincible. Modern diets (“modern” meaning after the agricultural revolution 10,000 years ago) are rich in carbohydrates, sucrose, and other sugars — much richer than what our hunter-gatherer ancestors experienced for hundreds of thousands of years. This new diet upset the well-balanced microbiome in our mouths, which are filled with around 700 kinds of bacteria. Some of these bacteria thrive on sugars, and left unchecked (i.e., without brushing, flossing, and regular dental visits), they attack enamel with lactic acid, a byproduct of the bacteria’s metabolism. So while enamel is the hardest stuff found throughout the human body, it needs extra special attention to keep all 32 teeth covered in it healthy and strong.

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In the fourth century BCE, Aristotle wrote in Parts of Animals that “the fact that human beings only are susceptible to tickling is due (1) to the fineness of their skin and (2) to their being the only creatures that laugh.” Although this ancient theory misses the mark, it lies at the beginning of a long tradition of philosophers and scientists questioning the nature of tickling. Thinkers such as René Descartes, Baruch Spinoza, Galileo, and Charles Darwin have provided their own theories about this weird autonomic response. Yet despite 2,500 years of investigating the matter, scientists still aren’t sure why it’s impossible to tickle yourself. 

Humans are the only animals that are ticklish.

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Humans stand apart from other animals in many ways, but being ticklish isn’t one of them. Homo sapiens’ closest relatives — bonobos, chimpanzees, and gorillas — all respond to tickling. Dogs, penguins, rats, and many other animals can also be tickled (metaphorically) pink.

A few theories about the nature of tickling have grown out of this legacy of scholarship. One idea is that tickling is a form of parental bonding, while another posits that it’s a warning mechanism to protect sensitive areas of our bodies from potential injury. We’ve also learned that there are actually two types of tickling: knismesis and gargalesis. Knismesis occurs when something light brushes across your skin, which is something you can induce by yourself. However, gargalesis — that uncontrollable funny-but-not-funny tickle — can only be triggered by another person. Why? Well, basically, your brain knows a tickle is coming when you do it to yourself, which in turn suppresses its response. Evidence from brain scans shows that the somatosensory cortex and the anterior cingulate cortex, which process touch and happy things, respectively, are more stimulated when tickled by someone else compared to ourselves. The only exception to this biological rule is people with schizophrenia, because they often can’t differentiate between self and external touch. Even though tickling usually induces a smile, just remember that those “happy feelings” during a tickle are actually part of an uncontrollable defense mechanism, not genuine laughter. So please — practice safe and consensual tickling. 

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Time seems like a simple enough concept — 60 seconds in a minute, 60 minutes in an hour, and so on. That is, except for a little something called “gravitational time dilation.” First explored in Albert Einstein’s general theory of relativity, the idea is almost confusingly simple — the farther away you are from a massive object (e.g., a planet), the faster time travels. The more massive the object, the slower time travels, which is why things get very wonky around supermassive black holes like the one at the center of our galaxy. 

Albert Einstein first proposed the idea of time travel.

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The concept of time travel was popularized by early science fiction. The first mention of it (at least as a voyage controlled by technology) comes from H.G. Wells, whose 1895 novel “The Time Machine” was published years before the arrival of Einstein’s famous theory of relativity.

These differences in how time flows are minuscule on Earth, so they don’t really affect us — the top floor of the Burj Khalifa, the world’s tallest building, does not operate with a special time-dilated clock. Yet technically, even our heads experience time just a bit differently than our feet. In 2010, the U.S. National Standards and Technology (NIST) even performed an experiment using optical atomic clocks that could measure a change in time dilation within less than 1 meter. 

Although imperceptible to our minds, precision technologies such as GPS need to factor in time dilation in order to work at all. So the next time you use Google Maps, consider giving a shout-out to Einstein and his mind-bending theory of the universe. 

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It’s likely that at some time in your life, a certain smell, whether the earthy aroma of freshly cut grass or the unmistakable fragrance of your grandparents’ house, triggered a powerful and strangely detailed memory. Well, there’s a biological reason for that. Unlike our four other best-known senses, whose electrical impulses are first sent to our thalamus before being sent to appropriate areas for memory, such as the hippocampus, our sense of smell takes a different route. Instead of being sent to the thalamus, scents go straight to the smell center known as the olfactory bulb. The fact that this bulb is directly connected to the hippocampus and the amygdala (which is responsible for emotional processing) is likely why smell evokes such powerful memories compared to our other senses. These memories can be extremely distinct, and they’re often linked to our childhood, likely because they were first stored when we experienced the scent at a young age.

Most of our sense of taste comes from our sense of smell.

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Smell is by far the most important facet of taste. The gustatory nerve cells located in taste buds can sense only sweet, sour, bitter, salty, and umami (savory). Because smell accounts for 80% of taste, without it, humans would be limited to only those five basic tastes.

According to a 2017 study, a part of the olfactory bulb known as the piriform cortex is what allows certain smells to be deposited in our long-term memory, though this cortex requires other parts of the brain to pull this off. The olfactory bulb essentially consults our orbitofrontal cortex, an area of the brain responsible for higher-level taste and smell functions, about whether a smell should be stored in long-term memory. 

Companies are very aware that smell can be a powerful reminder of memory and emotion, which is why some of them have even trademarked certain scents (yes, you can do that). Verizon, for example, owns the rights to the “flowery musk scent” used in its stores, and olfactive branding companies work with clients like Nike to leverage the power of smell. Because when it comes to unlocking human emotions and memories, it might be the strongest sense we have.

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Our diversity is part of what makes human beings special. Yet as far as our genes are concerned, we’re all fairly similar: Humans share 99.9% of their genes with one another. To put this into perspective, bonobos and chimpanzees — the closest relatives to humans in the animal kingdom — share approximately 98.8% of their genes with humans. Clearly, even small differences in genetic similarity can have a major impact. 

Modern humans still have traces of DNA from other human species.

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Some 70,000 years ago, at least four species of humans coexisted on Earth — Homo sapiens, Neanderthals, Homo floresiensis, and Denisovans. Evidence of this coexistence can be found in human genetics, with small percentages of Neanderthal and Denisovan DNA embedded in our genes.

That may be especially true when it comes to human health. According to the National Institutes of Health, nine of the 10 leading causes of death in the U.S. (barring accidental deaths) are influenced by our genetics, and variations among individuals can mean significantly varying health outcomes. 

In the 21st century, advances in our understanding of the human genome — thanks to the completion of groundbreaking scientific studies including the Human Genome Project — have pushed medicine into the genetic frontier. Now doctors can screen newborns for genetic abnormalities and sometimes use gene-based therapies, while nutritionists are using genomics to tailor diets to specific genetic dispositions. According to some, the future of medicine is in our genes.

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The human body has 206 bones — unless you’re talking about babies, in which case the number is closer to 300. Many of a newborn’s bones are actually made of cartilage, which is much more malleable and allows fetuses to curl inside the womb as they develop. As children grow, cartilage turns into bone in a process called ossification, and the excess bones fuse together. (If you’ve ever wondered how those “soft spots” on an infant’s head — technically known as fontanelles — become stronger, bone fusion is the answer.) This is also a big part of why calcium is so important for babies: New bone tissue can’t grow without it.

Newborns don’t cry tears.

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For the first weeks of their life, babies don’t technically cry. They may make a lot of noise when expressing their displeasure, but because tear ducts don’t fully form for a month or so, their eyes will remain dry while they do so.

Ossification doesn’t happen overnight, however — it continues until a person reaches their mid-20s, which is around when humans reach their peak bone mass. In much the same way that we’re constantly shedding our skin, our bones are constantly changing as well, with old bone gradually destroyed and new bone material formed. The process is called remodeling, and it helps keep the skeletal system healthy long after we’ve settled down at 206 bones.

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