What is déjà vu?

What exactly is déjà vu?

While people have wondered about déjà vu for a long time, only recently have scientists started experimentally investigating what might trigger it. Read this deep dive into the latest science.

Why do you feel like you've been here before?

Walking into a new environment and feeling like you've been there before is a hallmark of déjà vu, a French term meaning "already seen" that describes the sensation of remembering the unfamiliar and may come with "an illusory sense of prediction." Research suggests déjà vu may manifest when there's a hiccup in the brain's familiarity detection and memory recall, forcing one's attention inward to seek the supposed source of familiarity.

Déjà vu is a memory illusion involving brain regions correcting inaccurate memories

Neuroscientists suggest the phenomenon is your brain recognizing that this feeling of familiarity is incorrect, showing off the brain's internal fact-checking system. Younger people experience déjà vu more often because their brains have higher excitatory activity and stronger memory monitoring systems.

How science explains déjà vu

Take a fun look at the science explaining the odd experience of déjà vu, which some theories suggest is actually a sign of a healthy brain.

Voir dire is the process that determines who actually sits on a jury

Meaning "to speak the truth" in French, voir dire is the questioning of prospective jurors by a judge or attorney to assess their impartiality and suitability for service. Those deemed unfit are excused before a final jury is seated.

Vertigo is considered a subtype of dizziness

This two-minute video explains vertigo—a subtype of dizziness—and its many potential causes. It explores the vestibular system, the inner ear system, which typically governs our sense of stability, and details its anatomy.

There’s a psychological explanation for ‘FOMO,’ or the fear of missing out

This video breaks down FOMO—what it is, how it works, and why it happens—showing how social comparison and uncertainty trigger anxiety that pushes people toward risk-taking and constant checking behaviors.

Around 5% of people report memories of a near-death experience

Near-death experiences are moments reported by people who’ve come close to death, often associated with bright lights, out-of-body sensations, or encounters with otherworldly beings. While scientific research offers theories ranging from brain chemistry to psychological coping mechanisms, no single explanation fully captures the mystery of these experiences.

Why does that bag look like a face?

If you've ever spotted a face hidden in an inanimate object, you've engaged in a timeless phenomenon known as pareidolia. This three-minute video explains why and how it happens.

Synesthesia: when sensation in one sense triggers an unrelated sense in the brain

The first reported clinical case of synesthesia dates back to the early 1800s, though the perceptual phenomenon may be much older than surviving records imply. This article by researcher Richard Cytowic details the scientific history of synesthesia, a neurological trait that causes some senses to overlap, giving color to numbers and flavors to names.

What exactly is dopamine?

Dopamine is a neurotransmitter—a chemical released by neurons that enables neural communication—that makes people want things with an "almost animalistic desire," compelling them to snack or check phone notifications. Often misrepresented as a "pleasure" chemical, dopamine is involved in motivation, learning, attention, habit formation, and more. Trends like dopamine fasting overinflate its role in mental health and often use it as part of "straw man" biohacking claims.

Why do we sometimes jolt awake before falling asleep?

Around 70% of people are estimated to experience the sensation of falling and jerking awake right as they're about to drift to sleep. That mysterious phenomenon is called hypnic jerking. As common as it is, hypnotic jerking remains disconcerting and hard to explain. While we don't yet have clear answers, peruse this article in Popular Science for some possible explanations as to why this happens.

In the 'block universe' model, the past, present, and future exist simultaneously

According to special relativity, observers do not experience the same events in the same order, but all perspectives are valid. This means what is considered one person's as yet unexperienced future may have already taken place in someone else's past.

Researchers believe the brain consolidates memories, processes emotions, and plans for the future during dream states

What (and why) is ASMR?

ASMR podcasts and videos are appearing all over social media, so what exactly is it, and what does it do? This immersive article explains the phenomena of ASMR, which stands for autonomous sensory meridian response. ASMR is when certain sounds or...

Relative motion between a source and an observer shifts the color of observed light

The phenomenon, known as the Doppler effect, is the result of experiencing an increase in the frequency of observed waves when approaching a source and a decrease when moving away. Because this property corresponds to color, approaching objects appear bluer than normal—blueshift—and receding objects appear redder than usual—redshift.

Watch a video that visualizes what really happens inside a credit card network when you make a purchase.

Every tap sets off a lightning-fast relay. When someone buys coffee with their Visa, a tiny "runner" races between the coffee shop's bank, Visa's network, and their bank—all in under a second. The catch? No money has actually moved yet. Here's what really happens behind that instant "Approved."

1440: Everything you need to know about Alzheimer's

What is Alzheimer's, and is it different from dementia? In short, yes—Alzheimer's is a specific brain disease that falls under the umbrella of dementia. However it is the most prevalent form of the condition, accounting for 60 to 80% of reported dementia cases.

What is a heat dome?

The world is seeing regular heat waves with multiple causes, including so-called "heat domes," a term rising in use in recent years. A heat dome occurs when the atmosphere traps hot ocean air like a lid or cap, creating stifling conditions and resulting in extended periods of heat. See an illustration of this phenomenon and read how it works with this brief explainer.

How entanglement lies at the heart of quantum computing

Theoretical physicist and mathematician Frank Wilczek boils down the complex notion of quantum entanglement into this reasonably understandable summary. Wilczek discusses the concepts of quantum entanglement and how it relates to other ideas like the Many-Worlds Interpretation (MWI) of quantum mechanics. What sounds a bit "science-fictiony" can have actual real-world and concrete applications.

'Dystopia,' explained

John Stuart Mill introduced the term "dystopia" in 1868 to critique harmful policies, calling it the opposite of utopia, or an ideal world. The concept grew as industrialization and totalitarian regimes exposed societal flaws, inspiring works like George Orwell's "1984" to warn against oppression and moral decay.

What is mixed reality?

Mixed reality is a technology that blends the physical and digital worlds, allowing virtual and real objects to coexist and interact in real-time. It combines elements of both virtual reality and augmented reality, enabling users to perceive and manipulate digital content as if it were part of their physical environment.

Hear history nerd Ray Dalio talk about making decisions during times of uncertainty

For Dalio, history is the best teacher. Dalio emphasizes relying on and respecting the past as a framework for the future, so that during unsettling times, people still make good decisions.

The early art of Dalí represents peak surrealism

When it debuted in New York in 1932, "The Persistence of Memory" captured international audiences with its absurd, dream-like reflection on time. In his later works, Dalí's work evolved and became more of a self-parody. This video breaks down the great Spanish artist's works.

‘Demo days’ are when founders at Y Combinator pitch their startup ideas

Investors sit in the audience, waiting to hear what this season’s batch of YC founders have cooked up. YC argues that going through the process of a demo day can help founders get a better understanding of their startup themselves.

The history behind New Orleans' drive-thru daiquiris

New Orleans attracts millions of annual visitors with its freewheeling drinking culture, reaching its apex during the weeks-long Mardi Gras celebration. But even the most intemperate tourists might be confused by the city's drive-through daiquiri stands. This resource offers a brief history of the quirky staple and explains how it has survived changes in the law.

According to the physics of black holes, our 4D reality may be a hologram—a projection of information coded on a 2D surface on the edge of the universe.

Salvador Dalí believed he was his own dead brother, who had died nine months before he was born

The Dracula character, now seen as the vampire archetype, was inspired by a seafood-induced nightmare its creator experienced.

The concept of phantom limbs, the perception that amputated limbs are still present, dates back to a French military surgeon in the 16th century.

When a Paris department store hesitated to sell Estée Lauder's perfume, she 'accidentally' broke a bottle, causing the smell to spread throughout the store and shoppers to start asking to buy the scent.

Ebola is a filovirus, meaning it resembles a thread under a microscope, though it sometimes twists and can resemble a bowl of spaghetti.

A researcher recently discovered an interview from 1967 between groundbreaking journalist Joan Didion and the Grateful Dead, years before the band became an institution.

Long before Major League Baseball began allowing instant replay for balls and strikes, an umpire reluctantly used the technology to determine whether a hit was a home run, a move that inadvertently ushered in a new era for the sport.

Apeirophobia is an overwhelming fear of living forever, perhaps stemming from the brain's inability to conceptualize an infinite life.

Stars appear to twinkle because turbulence in the motion of molecules in Earth's atmosphere causes light to bend and wiggle rapidly on its way to the surface—a phenomenon called scintillation.

Venture capital is a form of funding provided to early-stage companies with high growth potential, typically in exchange for an equity (or ownership) stake

Some coin denominations have ridges etched on their sides, partly to prevent counterfeiting, in a practice called 'reeding.'

The first steps to rebuilding your credit score include getting to know exactly what's on your credit report and figuring out how far you are from the '700 club,' or 'good score' range.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Visualize how CRISPR-Cas9—a type of immune system used by single-celled organisms to attack invasive viruses—targets and makes precise changes to a gene's DNA.

For centuries, Niagara Falls has been a magnet for daredevils seeking to conquer one of nature's most powerful wonders, with its 200-foot drop and water rushing at 32 feet per second with 280 tons of force.

The rivalry between Alexander Hamilton and Thomas Jefferson centered on federalism—the question of how much power the federal government should hold relative to the states.

Dorothy's famous entrance into Oz in 'The Wizard of Oz' relied on analog tricks, including a body double and sepia-toned paint, as much as it did on technology

Browse a detailed visual analysis of over 200,000 similes—comparisons that often use 'like' or 'as'—from popular fiction to see how frequently writers the same idiomatic expressions.

This poison ivy knowledge quiz—accompanied by photos of the plant's varied forms—may help you identify this common rash-inducing growth.

The word 'jazz' evolved from 'jasm,' 19th-century American slang for 'energy'

Existentialist writer Jean-Paul Sartre coined the famous phrase 'Hell is—other people' in his play 'No Exit'

Discover the catastrophic consequences of a failure of the US electrical grid—something that is more likely than many Americans think.

See a 3D reconstruction of Tenochtitlan, the Aztec capital that was home to an estimated population of 200,000 to 400,000.