It’s 4am, and the big test is in 8 hours. You’ve been studying for days, but you still don’t feel ready. Should you drink another cup of coffee and spend the next few hours cramming? Or should you go to sleep? Shai Marcu defends the latter option, showing how sleep restructures your brain in a way that’s crucial for how our memory works. Lesson by Shai Marcu, animation by Javier Saldeña.

The human eye is an amazing mechanism, able to detect anywhere from a few photons to a few quadrillion, or switch focus from the screen in front of you to the distant horizon in a third of a second. How did these complex structures evolve? Joshua Harvey details the 500 million year story of the human eye. [Directed by Artrake Studio, narrated Addison Anderson, music by Bojana Vunturišević].

The wheels in your brain are constantly turning, even when you're asleep or not paying attention. In fact, most of your brain’s activities are ones you’d never be aware of … unless they suddenly stopped. Nathan S. Jacobs takes us inside the always active, surprisingly spontaneous brain. Lesson by Nathan S. Jacobs, animation by TOGETHER.

We tend to think of blindness as something you're born with, but with certain genetic diseases, it can actually develop when you’re a kid, or even when you’re an adult. But could blind eyes possibly regenerate? David Davila explains how the zebrafish’s amazing regenerative retinas are causing scientists to investigate that very question. Lesson by David Davila, animation by Eli Enigenburg.

GPS location apps on a smartphone can be very handy when mapping a travel route or finding nearby events. But how does your smartphone know where you are? Wilton L. Virgo explains how the answer lies 12,000 miles over your head, in an orbiting satellite that keeps time to the beat of an atomic clock powered by quantum mechanics. [Directed by Nick Hilditch, narrated by Addison Anderson].

You may have heard the expression that dogs 'see with their noses.' But these creature's amazing nasal architecture actually reveals a whole world beyond what we can see. Alexandra Horowitz illustrates how the dog's nose can smell the past, the future and even things that can't be seen at all. [Directed by Província Studio, narrated by Pen-Pen Chen].

Today, about one-tenth of the world's population are southpaws. Why are such a small proportion of people left-handed -- and why does the trait exist in the first place? Daniel M. Abrams investigates how the uneven ratio of lefties and righties gives insight into a balance between competitive and cooperative pressures on human evolution. [Directed by TED-Ed, narrated by Addison Anderson, music by Michael Dow].

Objects that fly faster than the speed of sound (like really fast planes) create a shock wave accompanied by a thunder-like noise: the sonic boom. These epic sounds can cause distress to people and animals and even damage nearby buildings. Katerina Kaouri details how scientists use math to predict sonic booms' paths in the atmosphere, where they will land, and how loud they will be. [Directed by Anton Bogaty, narrated by Addison Anderson, music by Michael Dow].

Nestled in the tissues of your neck is a small, unassuming organ that wields enormous power over your body: the thyroid. Emma Bryce explains how the thyroid, like the operations manager in a company, is tasked with making sure that all the cells in your body are working properly. [Directed by Tremendousness, narrated by Pen-Pen Chen].

Sitting down for brief periods can help us recover from stress or recuperate from exercise. But nowadays, our lifestyles make us sit much more than we move around. Are our bodies built for such a sedentary existence? Murat Dalkilinç investigates the hidden risks of sitting down. [Directed by Oxbow Creative, narrated by Addison Anderson].

The biggest challenge in a parasite’s life is to move from one host to another. Intriguingly, many parasites have evolved the ability to manipulate the behavior of their hosts to improve their own survival -- sometimes even by direct brain control. Jaap de Roode details a few parasites that can really mess with the mind. Lesson by Jaap de Roode, animation by Andrew Foerster.

When you think of Archimedes' Eureka moment, you probably imagine a man in a bathtub, right? As it turns out, there's much more to the story. Armand D'Angour tells the story of Archimedes' biggest assignment -- an enormous floating palace commissioned by a king -- that helped him find Eureka. [Directed by Zedem Media, narrated by Addison Anderson].

Twenty four hundred years ago, Plato, one of history’s most famous thinkers, said life is like being chained up in a cave forced to watch shadows flitting across a stone wall. Beyond sounding quite morbid, what exactly did he mean? Alex Gendler unravels Plato's Allegory of the Cave, found in Book VII of "The Republic." Lesson by Alex Gendler, animation by Stretch Films, Inc.

People have been grappling with the question of artificial creativity -- alongside the question of artificial intelligence -- for over 170 years. For instance, could we program machines to create high quality original music? And if we do, is it the machine or the programmer that exhibits creativity? Gil Weinberg investigates this creative conundrum. [Directed by TOGETHER, narrated by Addison Anderson, music by Mason Bretan, Nick Sewell and Shimon the Robot].

Halitosis is a curse that has plagued humanity since ancient times. But what causes it, and why is it so universally terrifying? Mel Rosenberg outlines the basics of bad breath — and what you can do when it strikes you. [Directed by Andrew Foerster, narrated by Addison Anderson, music by Devin Polaski].

When we measure things, most people are only worried about how accurate, or how close to the actual value, they are. Looking at the process of measurement more carefully, you will see that there is another important consideration: precision. Matt Anticole explains what exactly precision is and how can help us to measure things better. [Directed by Anton Bogaty, narrated by Addison Anderson].

Authentic dollar bills are equipped with many security features to make them difficult to forge. But that doesn’t stop counterfeiters from trying to fool people with fakes. Luckily, anyone can help catch a counterfeit; all you need are a few simple tools and a bit of chemistry. Tien Nguyen details the chemistry behind counterfeit cash. Lesson by Tien Nguyen, animation by Lippy.

It's so obvious that it's practically proverbial: you can't unboil an egg. But actually, it turns out that you can -- sort of. Eleanor Nelsen explains the process by which mechanical energy can undo what thermal energy has done. [Directed by Província Studio, narrated by Addison Anderson].

Imagine the brain could reboot, updating its damaged cells with new, improved units. That may sound like science fiction — but it’s a potential reality scientists are investigating right now. Ralitsa Petrova details the science behind neurogenesis and explains how we might harness it to reverse diseases like Alzheimer’s and Parkinson’s. Lesson by Ralitsa Petrova, animation by Artrake Studio.

Some people love the feeling of cracking their knuckles, while others cringe at the sound. But what causes that trademark pop? And is it dangerous? Eleanor Nelsen gives the facts behind joint popping. [Directed by Steve Belfer Creative, narrated by Addison Anderson].

Beginning around 1377, medieval England was shaken by a power struggle between two noble families, which spanned generations and involved a massive cast of characters, complex motives and shifting loyalties. Sound familiar? Alex Gendler illustrates how the historical conflict known as the Wars of the Roses served as the basis for much of the drama in Game of Thrones. [Directed by Brett Underhill, narrated by Addison Anderson, music by WORKPLAYWORK and Cem Misirlioglu].

Mansa Musa, the 14th century African king of the Mali Empire, is said to have amassed a fortune that possibly made him one of the wealthiest people who ever lived. Jessica Smith tells the story of how Mansa Musa literally put his empire – and himself – on the map. [Directed by Sandro Katamashvili, narrated by Pen-Pen Chen].

There's a common misconception that if you like to meticulously organize your things, keep your hands clean, or plan out your weekend to the last detail, you might be OCD. In fact, OCD (Obsessive Compulsive Disorder) is a serious psychiatric condition that is frequently misunderstood by society and mental health professionals alike. Natascha M. Santos debunks the myths surrounding OCD. [Directed by Zedem Media, narrated by Addison Anderson].

In 1997, Brazilian football player Roberto Carlos set up for a 35 meter free kick with no direct line to the goal. Carlos's shot sent the ball flying wide of the players, but just before going out of bounds it hooked to the left and soared into the net. How did he do it? Erez Garty describes the physics behind one of the most magnificent goals in the history of football. [Directed by TOGETHER, narrated by Addison Anderson, music by Nick Sewell].

One hundred green-eyed logicians have been imprisoned on an island by a mad dictator. Their only hope for freedom lies in the answer to one famously difficult logic puzzle. Can you solve it? Alex Gendler walks us through this green-eyed riddle. [Directed by Artrake Studio, narrated by Addison Anderson].

As anyone who’s seen a mummy knows, ancient Egyptian priests went to a lot of trouble to evade decomposition. But how successful were they? Len Bloch details the mummification process and examines its results thousands of years later. Lesson by Len Bloch, animation by The Moving Company Animation Studio.

Can you grow a human bone outside the human body? The answer may soon be yes. Nina Tandon explores the possibility by examining how bones naturally grow inside the body, and illuminating how scientists are hoping to replicate that process in a lab. Lesson by Nina Tandon, animation by Giant Animation Studios.

It may seem like the semicolon is struggling with an identity crisis. It looks like a comma crossed with a period. Maybe that's why we toss these punctuation marks around like grammatical confetti; we're confused about how to use them properly. Emma Bryce clarifies best practices for the semi-confusing semicolon. [Directed by Karrot Entertainment, narrated by Addison Anderson, music by Tom Jordan].

Over-the-counter pregnancy tests give potentially life-changing results with a pretty high rate of accuracy. But how do they work? Tien Nguyen explains how each test performs a scientifically rigorous, multi-stage experiment that goes from start to finish in the time that it’ll take you to watch this video. Lesson by Tien Nguyen, animation by Andrew Foerster.

We hear about calories all the time: How many calories are in this cookie? How many are burned by doing 100 jumping jacks, or long-distance running, or fidgeting? But what is a calorie, really? And how many of them do we actually need? Emma Bryce explains how a few different factors should go into determining the recommended amount for each person. [Directed by Qa'ed Mai, narrated by Addison Anderson].

Do animals think? It’s a question that has intrigued scientists for thousands of years, inspiring them to come up with different methods and criteria to measure the intelligence of animals. Bryan B Rasmussen navigates through this controversial question, showing how determining intelligence often says more about how humans think than about anything else. Lesson by Bryan B Rasmussen, animation by Mike Schell.

When we talk about 'English', we often think of it as a single language. But what do the dialects spoken in dozens of countries around the world have in common with each other, or with the writings of Chaucer? Claire Bowern traces the language from the present day back to its ancient roots, showing how English has evolved through generations of speakers. [Directed by Patrick Smith, narrated by Addison Anderson].

Has anyone ever told you, "Stand up straight!" or scolded you for slouching at a family dinner? Comments like that might be annoying—but they're not wrong. Your posture is the foundation for every movement your body makes and can determine how well your body adapts to the stresses on it. Murat Dalkilinç gives the pros of good posture. [Directed by Nadav Arbel, narrated by Addison Anderson].

Throughout the history of mankind, the subject of identity has sent poets to the blank page, philosophers to the agora and seekers to the oracles. These murky waters of abstract thinking are tricky to navigate, so it's probably fitting that to demonstrate the complexity, the Greek historian Plutarch used the story of a ship. Amy Adkins illuminates Plutarch's Ship of Theseus. [Directed by Stretch Films, Inc., narrated by Addison Anderson].

Tens of millions of years ago, plate tectonics set North and South America on an unavoidable collision course that would change the face of the Earth and spell life or death for thousands of species. Juan D. Carrillo explains the massive biological repercussions of this collision, which caused one of the greatest episodes of biological migration in history: The Great American Biotic Interchange. Lesson by Juan D. Carrillo, animation by TED-Ed.

Take a journey through Walt Whitman's poem 'A Noiseless Patient Spider' with the help of three animators who each used a different animation style to bring this beautiful poem to life. Lesson by Justin Moore, animation by TED-Ed.

What do Charles Darwin, Michael Jordan, and Yoda have in common? They, like many other historical and fictive individuals, are bald. Scientists have long pondered, why do some people lose their hair, and how can we bring it back? Sarthak Sinha explores the basics of baldness. [Directed by Brett Underhill, narrated by Addison Anderson].

In previous decades, most news with global reach came from several major newspapers and networks with the resources to gather information directly. The speed with which information spreads now, however, has created the ideal conditions for something called circular reporting. Noah Tavlin sheds light on this phenomenon. Lesson by Noah Tavlin, animation by Patrick Smith.

Taking that internship in a remote mountain lab might not have been the best idea. Pulling that lever with the skull symbol just to see what it did probably wasn't so smart either. But now is not the time for regrets because you need to get away from these mutant zombies...fast. Can you use math to get you and your friends over the bridge before the zombies arrive? Alex Gendler shows how. [Directed by Artrake Studio, narrated by Addison Anderson, music by WORKPLAYWORK and Cem Misirlioglu].

Pascal's triangle, which at first may just look like a neatly arranged stack of numbers, is actually a mathematical treasure trove. But what about it has so intrigued mathematicians the world over? Wajdi Mohamed Ratemi shows how Pascal's triangle is full of patterns and secrets. [Directed by Henrik Malmgren, narrated by Addison Anderson, music by Henrik Malmgren].

The Great Wall of China is a 13,000-mile dragon of earth and stone that winds its way through the countryside of China. As it turns out, the wall's history is almost as long and serpentine as its structure. Megan Campisi and Pen-Pen Chen detail the building and subsequent decay of this massive, impressive wall. [Directed by Steff Lee, narrated by Pen-Pen Chen, music by Gav Cantrell].

The constant thud underneath your feet. The constrained space. The monotony of going nowhere fast. Running on a treadmill can certainly feel like torture, but did you know it was originally used for that very purpose? Conor Heffernan details the dark and twisted history of the treadmill. Lesson by Conor Heffernan, animation by Yukai Du.

You and nine other individuals have been captured by super-intelligent alien overlords. The aliens think humans look quite tasty, but their civilization forbids eating highly logical and cooperative beings. Unfortunately, they're not sure whether you qualify, so they decide to give you all a test. Can you solve this hat riddle? Alex Gendler shows how. [Directed by Artrake Studio, narrated by Addison Anderson, music by WORKPLAYWORK and Cem Misirlioglu].

When faced with a big challenge where potential failure seems to lurk at every corner, you've probably heard the advice, "Be more confident!" But where does confidence come from, and how can you get more of it? Here are three easy tips to boost your confidence. [Directed by Kozmonot Animation Studio, narrated by Susan Zimmerman, music by WORKPLAYWORK and Cem Misirlioglu].

Did you know that gold is extraterrestrial? Instead of arising from our planet's rocky crust, it was actually cooked up in space and is present on Earth because of cataclysmic stellar explosions called supernovae. CERN Scientist David Lunney outlines the incredible journey of gold from space to Earth. [Directed by Andrew Foerster, narrated by Addison Anderson, music by Devin Polaski].

A handful of species on Earth share a seemingly mysterious trait: a menstrual cycle. We're one of the select few mammals on Earth that menstruate, and we also do it more than any other animal, even though it's a waste of nutrients, and can be a physical inconvenience. So where's the sense in this uncommon biological process? TED-Ed describes the history and evolution of menstruation. [Directed by TED-Ed, narrated by Susan Zimmerman].

Our hard-wired stress response is designed to give us the quick burst of heightened alertness and energy needed to perform our best. But stress isn't all good. When activated too long or too often, stress can damage virtually every part of our body. Sharon Horesh Bergquist gives us a look at what goes on inside our body when we are chronically stressed. [Directed by Adriatic Animation, narrated by Addison Anderson, music by Jure Buljević].

We have over 600 muscles in our bodies that help bind us together, hold us up, and help us move. Your muscles also need your constant attention, because the way you treat them on a daily basis determines whether they will wither or grow. Jeffrey Siegel illustrates how a good mix of sleep, nutrition and exercise keep your muscles as big and strong as possible. [Directed by Brett Underhill, narrated by Addison Anderson].

Stress isn't always a bad thing; it can be handy for a burst of extra energy and focus, like when you're playing a competitive sport or have to speak in public. But when it's continuous, it actually begins to change your brain. Madhumita Murgia shows how chronic stress can affect brain size, its structure, and how it functions, right down to the level of your genes. [Directed by Andrew Zimbelman, narrated by Addison Anderson, music by Josh Smoak].

Imagine if you could plug your brain into a machine that would bring you ultimate pleasure for the rest of your life. The only catch? You have to permanently leave reality behind. Hayley Levitt and Bethany Rickwald explore Robert Nozick's thought experiment that he called the Experience Machine. [Directed by Avi Ofer, narrated by Addison Anderson].

Before he turned physics upside down, a young Albert Einstein supposedly showed off his genius by devising a complex riddle involving a stolen exotic fish and a long list of suspects. Can you resist tackling a brain teaser written by one of the smartest people in history? Dan Van der Vieren shows how. [Directed by Artrake Studio, narrated by Addison Anderson, music by Dušan Strajnić].

Octopuses have the ability to solve puzzles, learn through observation, and even use tools – just like humans. But what makes octopus intelligence so amazing is that it comes from a biological structure completely different from ours. Cláudio L. Guerra takes a look inside the amazing octopus brain. [Directed by Cinematic, narrated by Addison Anderson].

When under anesthesia, you can't move, form memories, or — hopefully — feel pain. And while it might just seem like you are asleep for that time, you actually aren't. What's going on? Steven Zheng explains what we know about the science behind anesthesia. [Directed by Zedem Media, narrated by Addison Anderson].

Depression is the leading cause of disability in the world; in the United States, close to ten percent of adults struggle with the disease. But because it's a mental illness, it can be a lot harder to understand than, say, high cholesterol. Helen M. Farrell examines the symptoms and treatments of depression, and gives some tips for how you might help a friend who is suffering. [Directed by Artrake Studio, narrated by Addison Anderson].

One could say that snowflakes are simply frozen water — but if you compare a snowflake to an ice cube, you’ll notice a big difference. Why are all snowflakes six-sided? Why are none of them exactly the same? And how do we ski on them? Maruša Bradač sheds light on the secret life of snowflakes. Lesson by Maruša Bradač, animation by bottomless well films.