Chances are you’ve knocked on wood in the past month. But, really, why? Here’s one origin story:

Knocking on wood is thought to come from the folklore of the ancient Indo-Europeans, or possibly people who predated them, who believed that trees were home to various spirits.

Touching a tree would invoke the protection or blessing of the spirit within.

Somehow, this tradition has survived long after belief in these spirits had faded away.

To learn more about superstitions, watch this TED-Ed Lesson: Where do superstitions come from?

Art credit: TED-Ed/@jefflebars

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How much do you know about the tiny, mighty bees? Here are 8 facts worth learning about our formidable friends:

1. Bees make our surroundings beautiful. In addition to pollinating our crops, bees are responsible for pollinating all of the things that make spring sing. And they’re no novices — they’ve been producing honey from flowering trees (fruit trees, nut trees, and beyond) for 10-20 million years! [Source: The case of the vanishing honeybees.]

2. Bees are social insects. Honey bees live together in large, well-organized family groups and engage in a variety of complex tasks not practiced by solitary insects. Communication, complex nest construction, environmental control, defense, and division of the labor are just some of the behaviors that honey bees have developed to exist successfully in social colonies. And they are not the least bit lazy: one single bee colony can pollinate 300 million flowers each day. [Source: The case of the vanishing honeybees.]

3. Bees are above words. They communicate through ‘dance’ and pheromones. By performing what’s referred to as the ‘waggle dance’, bees can share information about the direction and distance to patches of flowers yielding nectar and pollen, to water sources, or to new nest-site locations. [Source: Why do honeybees love hexagons?]

4. Bees make great wingmen. Bees are very busy little matchmakers. The bees’ side of the whole “birds and the bees” business is to help plants find mates and reproduce. Today, around 170,000 plant species receive pollination services from more than 200,000 pollinator species, a good many of which are bees! In return, flowering plants are an abundant and diverse food source for pollinators. For instance, fossil records suggest that bees may have evolved from wasps that gave up hunting after they acquired a taste for nectar. [Source: How bees help plants have sex.]

5. Bees put food on our tables. Bees pollinate crops on an industrial scale, generating over one-third of US food production. Their work alone has contributed an estimated $15-20 billion of value to the US agricultural business. [Source: The case of the vanishing honeybees.]

6. Bees can totally pack up a car better than you. Honeybees are some of nature’s finest mathematicians. Not only can they calculate angles and comprehend the roundness of the earth, these smart insects build and live in one of the most mathematically efficient architectural designs around: the beehive. Charles Darwin himself wrote that the honeycomb is a masterpiece of engineering. It is “absolutely perfect in economizing labor and wax.” [Source: Why do honeybees love hexagons?]

7. Bees are hooked on coffee, too. When bees pollinate coffee plants, they consume low doses of caffeine from the coffee flower nectar, which means that bees are **BUZZZZZING** from a caffeine high just like us, AND helping us to get our coffee fix on the daily! [Source: The case of the vanishing honeybees.]

8. Honeybees are disappearing at astonishing rates. Not to be a **buzzkill**, but here’s a not-so-fun fact. In the past decade, the US honeybee population has been decreasing at an alarming and unprecedented rate. Bee mortality rates in commercial production have more than doubled in the last decade, and in 2015, 40% of bee colonies were reported lost in just a single year. There are a variety of factors causing Colony Collapse Disorder, and scientists everywhere are working to prevent further loss of bees. Keep reading to see how you can help. [Source: The case of the vanishing honeybees.]

Love bees as much as we do? Well, let’s give the bees a hand, for real! Plant some bee-friendly flowers and remember, when bees have access to good nutrition, we have access to good nutrition, through their pollination services.

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This is the Bop. The Bop is a type of social dance. Dance is a language, and social dance is an expression that emerges from a community. Below, Camille A. Brown offers a brief history of African-American social dance.

A social dance isn’t choreographed by any one person. It can’t be traced to any one moment. Each dance has steps that everyone can agree on, but it’s about the individual and their creative identity. Because of that, social dances bubble up, they change, and they spread like wildfire. They are as old as our remembered history. In African-American social dances, we see over 200 years of how African and African-American traditions influenced history. The present always contains the past. And the past shapes who we are and who we will be.

Now, social dance is about community and connection; if you knew the steps, it meant you belonged to a group. But what if it becomes a worldwide craze? Enter the Twist.

It’s no surprise that the Twist can be traced back to the 19th century, brought to America from the Congo during slavery. But in the late ‘50s, right before the Civil Rights Movement, the Twist is popularized by Chubby Checker and Dick Clark. Suddenly, everybody’s doing the Twist: white teenagers, kids in Latin America. It makes its way into songs and movies. Through social dance, the boundaries between groups become blurred.

The story continues in the 1980s and ’90s. Along with the emergence of hip-hop, African-American social dance took on even more visibility, borrowing from its long past, shaping culture and being shaped by it. Today, these dances continue to evolve, grow and spread.

Why do we dance? To move, to let loose, to express.

Why do we dance together? To heal, to remember, to say: “We speak a common language. We exist and we are free.”

Camille A. Brown is a choreographer fusing dance and social commentary to explore race, sexuality and femininity.

Title Design by Kozmonot Animation Studio

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In the third act of “Swan Lake”, the Black Swan pulls off a seemingly endless series of turns, bobbing up and down on one pointed foot and spinning around and around and around … thirty-two times. It’s one of the toughest sequences in ballet, and for those thirty seconds or so, she’s like a human top in perpetual motion. Those spectacular turns are called fouettés, which means “whipped” in French, describing the dancer’s incredible ability to whip around without stopping. Below, Arleen Sugano explains the physics of this famous ballet move.

The dancer starts the fouetté by pushing off with her foot to generate torque. But the hard part is maintaining the rotation. As she turns, friction between her pointe shoe and the floor, and somewhat between her body and the air, reduces her momentum. So how does she keep turning? Between each turn, the dancer pauses for a split second and faces the audience. Her supporting foot flattens, and then twists as it rises back onto pointe, pushing against the floor to generate a tiny amount of new torque.

At the same time, her arms sweep open to help her keep her balance. The turns are most effective if her center of gravity stays constant, and a skilled dancer will be able to keep her turning axis vertical.

The extended arms and torque-generating foot both help drive the fouetté. But the real secret and the reason you hardly notice the pause is that her other leg never stops moving. During her momentary pause, the dancer’s elevated leg straightens and moves from the front to the side, before it folds back into her knee.

By staying in motion, that leg is storing some of the momentum of the turn. When the leg comes back in towards the body, that stored momentum gets transferred back to the dancer’s body, propelling her around as she rises back onto pointe.

As the ballerina extends and retracts her leg with each turn, momentum travels back and forth between leg and body, keeping her in motion.

In Tchaikovsky’s ballet, the Black Swan is a sorceress, and her 32 captivating fouettés do seem almost supernatural. But it’s not magic that makes them possible. It’s physics.

Animation by Dancing Line Productions/TED-Ed

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Prolonged space travel takes a severe toll on the human body. As we seriously consider the human species becoming space-faring, a big question stands: even if we do break free from Earth’s orbit and embark on long-duration journeys among the stars, can we adapt to the extreme environments of space? Lisa Nip examines our odds.

Without an atmospheric barrier and a magnetic field like Earth’s, most planets and moons are bombarded with dangerous subatomic particles, like ionizing radiation.

These particles can pass through nearly anything and would cause potentially cancerous DNA damage to space explorers. So, to survive as a species during space travel, we’d have to develop methods to quickly program protective abilities into ourselves. A beta version of these methods is gene therapy, which we can currently use to correct genetic diseases.

Now, what if we could turn the tables on radiation? Human skin produces a pigment called melanin that protects us from the filtered radiation on Earth.

Melanin exists in many forms across species, and some melanin-expressing fungi use the pigment to convert radiation into chemical energy. Instead of trying to shield the human body, or rapidly repair damage, we could potentially engineer humans to adopt and express these fungal, melanin-based energy-harvesting systems. They’d then convert radiation into useful energy while protecting our DNA. This sounds pretty sci-fi, but may actually be achievable with current technology.

Check out what else scientists have up their sleeves in the TED-Ed Lesson: Could we survive prolonged space travel?

Animation by Bassam Kurdali/TED-Ed

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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? Below, Erez Garty describes the physics behind one of the most magnificent goals in the history of football.

According to Newton’s first law of motion, an object will move in the same direction and velocity until a force is applied on it. When Carlos kicked the ball he gave it direction and velocity, but what force made the ball swerve and score one of the most magnificent goals in the history of the sport?

The trick was in the spin. Carlos placed his kick at the lower right corner of the ball, sending it high and to the right, but also rotating around its axis.

The ball started its flight in an apparently direct route, with air flowing on both sides and slowing it down. On one side, the air moved in the opposite direction to the ball’s spin, causing increased pressure, while on the other side—the air moved in the same direction as the spin, creating an area of lower pressure.

That difference made the ball curve towards the lower pressure zone. This phenomenon is called the Magnus effect.

This type of kick, often referred to as a banana kick, is attempted regularly, and it is one of the elements that makes “the beautiful game” beautiful.

But curving the ball with the precision needed to both bend around the wall, and back into the goal is difficult. Too high and it soars over the goal. Too low and it hits the ground before curving. Too wide and it never reaches the goal.

Not wide enough and the defenders intercept it. Too slow and it hooks too early or not at all. Too fast and it hooks too late.

The same physics make it possible to score another apparently impossible goal — an unassisted corner kick.

The Magnus effect was first documented by Sir Isaac Newton after he noticed it while playing a game of tennis back in 1670. It also applies to golf balls, Frisbees and baseballs. In every case the same thing happens: the ball’s spin creates a pressure differential in the surrounding airflow that curves it in the direction of the spin.

And here’s a question: could you theoretically kick a ball hard enough to make it boomerang all the way around back to you? Sadly, no. Even if the ball didn’t disintegrate on impact or hit any obstacles, as the air slowed it, the angle of its deflection would increase, causing it to spiral into smaller and smaller circles until finally stopping. And just to get that spiral you’d have to make the ball spin over 15 times faster than Carlos’s immortal kick. So good luck with that.

Watch the full TED-Ed Lesson: The “impossible” free kick

Animation by TOGETHER/TED-Ed

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Sleep is critical for mind and body health. Without it, the effects can be severe. But what if you suffer from insomnia? Below, neuroscientist Claudia Aguirre provides 7 healthy tips for a better night’s sleep:

1. Aim for power hours. Sleep the recommended amount for a restorative night. That is: between 9 and 12 hours for school-aged children, 8 to 10 hours for teenagers, and 7 to 9 hours for adults. [Animation by TED-Ed]

2. Ban the blue. Filter the blue light of your electronic device and sleep better. Studies show that blue light from electronic devices can delay sleep onset and affect overall circadian rhythm. [Animation by Javier Saldeña/TED-Ed]

3. Spoon. Sleeping on the side may help the brain clear waste more efficiently than sleeping on the back or belly. [Animation by TED-Ed]

4. Breathe deep. Deep breathing triggers the body’s relaxation response. What’s more, inhaling can drive cerebrospinal fluid flow to help clear brain waste and oxygenate the brain. [Animation by TED-Ed]

5. Don’t overdo it. Science is still working this one out, but there are some cases where too much sleep can pose a health risk. Better set that alarm. [Animation by Alan Foreman/TED-Ed]

6. Exercise. Lab experiments show that regular exercise can protect the brain from sleep deprivation-induced memory deficits. [Animation by Andrew Zimbelman/TED-Ed]

7. Keep cool. You just might get a better night’s rest if you sleep in a cool room (or stick your feet out). [Animation by TED-Ed]

For more health tips from experts, check out 7 TED-Ed Lessons for a healthier you.

Author bio: Claudia Aguirre is a neuroscientist and the author of several TED-Ed Lessons, including What would happen if you didn’t sleep? and Does stress cause pimples?

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They’re cute, they’re lovable, and judging by the 26 billion views on over 2 million YouTube videos of them pouncing, bouncing, climbing, cramming, stalking, clawing, chattering and purring; one thing is certain: cats are very entertaining. But their strange feline behaviors, both amusing and baffling, leave many of us asking: Why do cats do that? Below, Tony Buffington provides 10 reasons for some of your cat’s strangest behaviors:

1. Throughout time, cats were simultaneously solitary predators of smaller animals and prey for larger carnivores. As predators, they hunted and killed to eat. As prey, they hid and escaped to survive.

2. Cats today retain many of the same instincts that allowed them to thrive in the wild for millions of years. These instincts are behind some of their weirdest behaviors.

3. In the wild, cats needed sharp claws for climbing, hunting and self-defense.

4. Sharpening their claws on nearby surfaces kept them conditioned and ready, helped stretch their back and leg muscles, and relieved some stress too.

5. So, it’s not that your house cat hates your couch, chair, ottoman, pillows, curtains and everything thing else you put in her environment. She’s ripping these things to shreds and keeping her claws in tip top shape because this is exactly what her ancestors did in order to survive.

6. As predators, cats are opportunistic and hunt whenever prey is available. Since most cat prey are small, cats in the wild needed to eat many times each day, and use a ‘stalk, pounce, kill, eat strategy’ to stay fed.

7. This is why your house cat prefers to chase and pounce on little toys and eat small meals over the course of the day and night.

8. In the wild, small prey tend to hide in tiny spaces in their natural environments. So one explanation for your cat’s propensity to reach into containers and openings is that she is compelled by the same curiosity that helped ensure the continuation of her species for millions of years before.

9. As animals that were preyed upon, cats evolved to “not get caught” and in the wild, the cats that were the best at avoiding predators thrived. It also explains why she prefers a clean and odor free litter box: that’s less likely to give away her location to any predators that may be sniffing around nearby.

10. Your house cat doesn’t need these particular skills to find and hunt down dinner in her food bowl today. But instinctually, viewing the living room from the top of the bookcase, is exactly what she has evolved to do.

To cats, our homes are their jungles. But if this is the case, in our own cat’s eyes, who are we? Big, dumb, hairless cats competing with them for resources? Terribly stupid predators they’re able to outsmart everyday? Or maybe they think we’re the prey?

Watch the full TED-Ed Lesson: Why do cats act so weird?

Animation by Chintis Lundgren/TED-Ed

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For most of history, interpretation was mainly done consecutively, with speakers and interpreters making pauses to allow each other to speak. But after the advent of radio technology, a new simultaneous interpretation system was developed in the wake of World War II. In the simultaneous mode, interpreters instantaneously translate a speaker’s words into a microphone while he or she speaks, without pauses. Those in the audience can choose the language in which they want to follow. On the surface it all looks seamless, but behind the scenes, human interpreters work incessantly to ensure every idea gets across as intended. And that is no easy task. Below, Ewandro Magalhaes explains how it works. [Learn about TED Translators here.]

It takes about two years of training for already fluent bilingual professionals to expand their vocabulary and master the skills necessary to become a conference interpreter. To get used to the unnatural task of speaking while they listen, students shadow speakers and repeat their every word exactly as heard, in the same language. In time, they begin to paraphrase what is said, making stylistic adjustments as they go. At some point a second language is introduced. Practicing in this way creates new neural pathways in the interpreter’s brain and the constant effort of reformulation gradually becomes second nature.

Over time, and through much hard work, the interpreter masters a vast array of tricks to keep up with speed, deal with challenging terminology and handle a multitude of foreign accents. They may resort to acronyms to shorten long names, choose generic terms over specific, or refer to slides and other visual aids. They can even leave a term in the original language while they search for the most accurate equivalent.

Interpreters are also skilled at keeping aplomb in the face of chaos. Remember: they have no control over who is going to say what or how articulate the speaker will sound. A curve ball can be thrown at any time. Also, they often perform to thousands of people and in very intimidating settings, like the UN General Assembly. To keep their emotions in check, they carefully prepare for an assignment, building glossaries in advance, reading voraciously about the subject matter, and reviewing previous talks on the topic.

Finally, interpreters work in pairs. While one colleague is busy translating incoming speeches in real time, the other gives support by locating documents, looking up words and tracking down pertinent information. Because simultaneous interpretation requires intense concentration, every 30 minutes the pair switches roles. Success is heavily dependent on skillful collaboration.

Watch the full TED-Ed Lesson: How interpreters juggle two languages at once:

Animation by @rewfoe/TED-Ed

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Ah, romantic love; beautiful and intoxicating, heart-breaking and soul-crushing… often all at the same time! Why do we choose to put ourselves though its emotional wringer? Does love make our lives meaningful, or is it an escape from loneliness and suffering? If romantic love has a purpose, neither science nor psychology has discovered it yet — but over the course of history, some of our most respected philosophers have put forward some intriguing theories. Below, Skye C. Cleary outlines five of these philosophical perspectives on why we love:

1. Love makes us whole, again / Plato (427—347 BCE)

The ancient Greek philosopher Plato explored the idea that we love in order to become complete. In his Symposium, he wrote about a dinner party at which Aristophanes, a comic playwright, regales the guests with the following story. Humans were once creatures with four arms, four legs, and two faces. One day they angered the gods, and Zeus sliced them all in two. Since then, every person has been missing half of him or herself. Love is the longing to find a soul mate who will make us feel whole again… or at least that’s what Plato believed a drunken comedian would say at a party.

2. Love tricks us into having babies / Schopenhauer (1788-1860)

Much, much later, German philosopher Arthur Schopenhauer maintained that love, based in sexual desire, was a “voluptuous illusion”. He suggested that we love because our desires lead us to believe that another person will make us happy, but we are sorely mistaken. Nature is tricking us into procreating and the loving fusion we seek is consummated in our children. When our sexual desires are satisfied, we are thrown back into our tormented existences, and we succeed only in maintaining the species and perpetuating the cycle of human drudgery. Sounds like somebody needs a hug.

3. Love is escape from our loneliness / Russell (1872-1970)

According to the Nobel Prize-winning British philosopher Bertrand Russell, we love in order to quench our physical and psychological desires. Humans are designed to procreate; but, without the ecstasy of passionate love, sex is unsatisfying. Our fear of the cold, cruel world tempts us to build hard shells to protect and isolate ourselves. Love’s delight, intimacy, and warmth helps us overcome our fear of the world, escape our lonely shells, and engage more abundantly in life. Love enriches our whole being, making it the best thing in life.

4. Love is a misleading affliction / Buddha (~6th- 4thC BCE)

Siddhartha Gautama, who became known as ‘the Buddha’, or ‘the enlightened one’, probably would have had some interesting arguments with Russell. Buddha proposed that we love because we are trying to satisfy our base desires. Yet, our passionate cravings are defects, and attachments — even romantic love — are a great source of suffering. Luckily, Buddha discovered the eight-fold path, a sort of program for extinguishing the fires of desire so that we can reach ‘nirvana’ — an enlightened state of peace, clarity, wisdom, and compassion.

5. Love lets us reach beyond ourselves / Beauvoir (1908-86)

Let’s end on a slightly more positive note. The French philosopher Simone de Beauvoir proposed that love is the desire to integrate with another and that it infuses our lives with meaning. However, she was less concerned with why we love and more interested in how we can love better. She saw that the problem with traditional romantic love is it can be so captivating that we are tempted to make it our only reason for being. Yet, dependence on another to justify our existence easily leads to boredom and power games.

To avoid this trap, Beauvoir advised loving authentically, which is more like a great friendship: lovers support each other in discovering themselves, reaching beyond themselves, and enriching their lives and the world, together.

Though we might never know why we fall in love, we can be certain that it’ll be an emotional rollercoaster ride. Maybe we lose ourselves. Maybe we find ourselves. It might be heartbreaking, or it might just be the best thing in life. Will you dare to find out?

Watch the full TED-Ed Lesson: Why do we love? A philosophical inquiry:

Animation by Avi Ofer/TED-Ed

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