Did you sign up to keep your brain in shape this summer — and learn 31 new ideas in the month of July? If so, you’re in great company. Around the world, more than 10,000 of you are taking the TED-Ed July Challenge at this very moment. Keep up the great work, challengers! Below, catch up on any lessons you missed with the TED-Ed July Challenge lesson archive. We’ll be updating this post regularly with more archive lessons as the TED-Ed July Challenge continues to unfold.

TED-Ed July Challenge lesson archive (2015)

July 1: How sugar affects the brain
When you eat something loaded with sugar, your taste buds, your gut and your brain all take notice. This activation of your reward system is not unlike how bodies process addictive substances such as alcohol or nicotine — an overload of sugar spikes dopamine levels and leaves you craving more. Nicole Avena explains why sweets and treats should be enjoyed in moderation. Take the lesson here »

July 2: How many universes are there?
The fact that no one knows the answer to this question is what makes it exciting. The story of physics has been one of an ever-expanding understanding of the sheer scale of reality, to the point where physicists are now postulating that there may be far more universes than just our own. Chris Anderson explores the thrilling implications of this idea. Take the lesson here »

July 3: The Oxford comma debate
If you read “Bob, a DJ and a clown” on a guest list, are three people coming to the party, or only one? That depends on whether you’re for or against the Oxford comma — perhaps the most hotly contested punctuation mark of all time. When do we use one? Can it really be optional, or is there a universal rule? TED-Ed explores both sides of this comma conundrum. Take the lesson here »

July 4: What you might not know about the Declaration of Independence
In June 1776, a little over a year after the start of the American Revolutionary War, the US Continental Congress huddled together in a hot room in Philadelphia to talk independence. Kenneth C. Davis dives into some of the lesser known facts about the process of writing the Declaration of Independence and questions one very controversial omission. Take the lesson here »

July 5: What makes tattoos permanent?
The earliest recorded tattoo was found on a Peruvian mummy in 6,000 BC. That’s some old ink! And considering humans lose roughly 40,000 skin cells per hour, how do these markings last? Claudia Aguirre details the different methods, machines and macrophages (you’ll see) that go into making tattoos stand the test of time. Take the lesson here »

July 6: Why sitting is bad for you
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. Take the lesson here »

July 7: How simple ideas lead to scientific discoveries
Adam Savage walks through two spectacular examples of profound scientific discoveries that came from simple, creative methods anyone could have followed — Eratosthenes’ calculation of the Earth’s circumference around 200 BC and Hippolyte Fizeau’s measurement of the speed of light in 1849. Take the lesson here »

July 8: Why do your knuckles pop?
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. Take the lesson here »

July 9: How playing an instrument benefits your brain
When you listen to music, multiple areas of your brain become engaged and active. But when you actually play an instrument, that activity becomes more like a full-body brain workout. What’s going on? Anita Collins explains the fireworks that go off in musicians’ brains when they play, and examines some of the long-term positive effects of this mental workout. Take the lesson here »

July 10: A glimpse of teenage life in ancient Rome
Welcome to the world of Lucius Popidius Secundus, a 17-year old living in Rome in 73 AD. His life is a typical one of arranged marriages, coming-of-age festivals, and communal baths. Take a look at this exquisitely detailed lesson on life of a typical Roman teenager two thousand years ago. Take the lesson here »

July 11: What is love?
Is love a signal winding through your neural pathways? A cliche? A cult? Love is easy to compare but difficult to define, maybe because we’re fundamentally biased; we try to define love while falling in or out of it. And love feels differently to every person who feels it, but this subjective emotion has evolutionary explanations, too. Brad Troeger takes a shot at the definition of love. Take the lesson here »

July 12: The great conspiracy against Julius Caesar
On March 15th, 44 BCE, Roman dictator Julius Caesar was assassinated by a group of about 60 of his own senators. Why did these self-titled Liberators want him dead? And why did Brutus, whose own life had been saved by Caesar, join in the plot? Kathryn Tempest investigates the personal and political assassination of Julius Caesar. Take the lesson here »

July 13: What causes bad breath?
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. Take the lesson here »

July 14: The Infinite Hotel Paradox
The Infinite Hotel, a thought experiment created by German mathematician David Hilbert, is a hotel with an infinite number of rooms. Easy to comprehend, right? Wrong. What if it’s completely booked but one person wants to check in? What about 40? Or an infinitely full bus of people? Jeff Dekofsky solves these heady lodging issues using Hilbert’s paradox. Take the lesson here »

July 15: How do batteries work?
Batteries are a triumph of science—they allow smartphones and other technologies to exist without anchoring us to an infernal tangle of power cables. Yet even the best batteries will diminish daily, slowly losing capacity until they finally die. Why does this happen, and how do our batteries even store so much charge in the first place? Adam Jacobson gives the basics on batteries. Take the lesson here »

July 16: The complex geometry of Islamic design
In Islamic culture, geometric design is everywhere: you can find it in mosques, madrasas, palaces, and private homes. And despite the remarkable complexity of these designs, they can be created with just a compass to draw circles and a ruler to make lines within them. Eric Broug covers the basics of geometric Islamic design. Take the lesson here »

July 17: The art of the metaphor
How do metaphors help us better understand the world? And, what makes a good metaphor? Explore these questions with writers like Langston Hughes and Carl Sandburg, who have mastered the art of bringing a scene or emotion to life. Take the lesson here »

July 18: Should we eat bugs?
What’s tasty, abundant and high in protein? Bugs! Although less common outside the tropics, entomophagy, the practice of eating bugs, was once extremely widespread throughout cultures. You may feel icky about munching on insects, but they feed about 2 billion people each day (Mmm, fried tarantulas). They also hold promise for food security and the environment. Emma Bryce makes a compelling case for dining on bugs. Take the lesson here »

July 19: A clever way to estimate enormous numbers
Have you ever tried to guess how many pieces of candy there are in a jar? Or tackled a mindbender like: “How many piano tuners are there in Chicago?” Physicist Enrico Fermi was very good at problems like these — learn how he used the power of 10 to make amazingly fast estimations of big numbers. Take the lesson here »

July 20: The cockroach beatbox
By dissecting a cockroach … yes, live on stage … TED Fellow and neuroscientist Greg Gage shows how brains receive and deliver electric impulses — and how legs can respond. (Launching a series on Awesome Nature) “The Cockroach Beatbox” was animated by the TED-Ed Animation Team (Jeremiah Dickey, Biljana Labovic, Celeste Lai, Kari Mulholland and Franz Palomares). Take the lesson here »

July 21: Why are some people left-handed
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. Take the lesson here »

July 22: What are those floaty things in your eye?
Sometimes, against a uniform, bright background such as a clear sky or a blank computer screen, you might see things floating across your field of vision. What are these moving objects, and how are you seeing them? Michael Mauser explains the visual phenomenon that is floaters. Take the lesson here »

July 23: How to unboil an egg
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. Take the lesson here »

July 24: How do we smell?
An adult human can distinguish up to 10,000 odors. You use your nose to figure out what to eat, what to buy and even when it’s time to take a shower. But how do the molecules in the air get translated into smells in your brain? Rose Eveleth charts the smelly journey through your olfactory epithelium and explains why scent can be so subjective. Take the lesson here »

July 25: A brief history of melancholy
If you are a living, breathing human being, chances are you have felt sad at least a few times in your life. But what exactly is melancholy, and what (if anything) should we do about it? Courtney Stephens details our still-evolving understanding of sadness — and even makes a case for its usefulness. Take the lesson here »

July 26: The chemistry of cookies
You stick cookie dough into an oven, and magically, you get a plate of warm, gooey cookies. Except it’s not magic; it’s science. Stephanie Warren explains via basic chemistry principles how the dough spreads out, at what temperature we can kill salmonella, and why that intoxicating smell wafting from your oven indicates that the cookies are ready for eating. Take the lesson here »

July 27: The loathsome, lethal mosquito
Everyone hates mosquitos. Besides the annoying buzzing and biting, mosquito-borne diseases like malaria kill over a million people each year (plus horses, dogs and cats). And over the past 100 million years, they’ve gotten good at their job — sucking up to three times their weight in blood, totally undetected. So shouldn’t we just get rid of them? Rose Eveleth shares why scientists aren’t sure. Take the lesson here »

July 28: Why is yawning contagious?
When you eat something loaded with sugar, your taste buds, your gut and your brain all take notice. This activation of your reward system is not unlike how bodies process addictive substances such as alcohol or nicotine — an overload of sugar spikes dopamine levels and leaves you craving more. Nicole Avena explains why sweets and treats should be enjoyed in moderation. Take the lesson here »

July 29: How tsunamis work
The immense swell of a tsunami can grow up to 100 feet, hitting speeds over 500 mph — a treacherous combination for anyone or anything in its path. Alex Gendler details the causes of these towering terrors and explains how scientists are seeking to reduce their destruction in the future. Take the lesson here »

July 30: Questions no one knows the answers to
In the first of the TED-Ed series videos designed to catalyze curiosity, TED Curator Chris Anderson shares his boyhood obsession with quirky questions that seem to have no answers. Take the lesson here »

July 31: How to build a fictional world
Why is J.R.R. Tolkien’s Lord of the Rings trilogy so compelling? How about The Matrix or Harry Potter? What makes these disparate worlds come alive are clear, consistent rules for how people, societies — and even the laws of physics — function in these fictional universes. Author Kate Messner offers a few tricks for you, too, to create a world worth exploring in your own words. Take the lesson here »

Check back for more lessons from the TED-Ed July Challenge. To find out about future TED-Ed Challenges, sign up for the TED-Ed weekly newsletter here >>