In the year leading up to this talk, the web tool Twitter exploded in size (up 10x during 2008 alone). Co-founder Evan Williams reveals that many of the ideas driving that growth came from unexpected uses invented by the users themselves.

Data can be used for good, harnessed for the betterment of society, but it can also be abused. Find out about some hidden not-so-sunny uses of Big Data.

How can we measure what makes a school system work? Andreas Schleicher walks us through the PISA test, a global measurement that ranks countries against one another -- then uses that same data to help schools improve. Watch to find out where your country stacks up, and learn the single factor that makes some systems outperform others.

Rather than demo another new technology, Tom Wujec reaches back to one of our earliest but most ingenious devices -- the astrolabe. With thousands of uses, from telling time to mapping the night sky, this old tech reminds us that the ancient can be as brilliant as the brand-new.

French street artist JR uses his camera to show the world its true face, by pasting photos of the human face across massive canvases. At TED2011, he makes his audacious TED Prize wish: to use art to turn the world inside out.

Shlomo Benartzi uses behavioral economics to study how and why we plan well for the future (or fail to), and uses that to develop new programs to encourage saving for retirement.

About this talk: Producer, songwriter, beatboxer and vocal arranger MaJiKer uses his whole body to express himself musically. Here, he premieres an experimental new piece that combines piano (occasionally played with his foot and head) with beatboxing, to wild effect.

Aditi Gupta uses storytelling and art to educate young girls about menstruation.

Stephen DeBerry uses the economy and culture to build a more just and joyful society.

Behavioral economist Dan Ariely, the author of Predictably Irrational, uses classic visual illusions and his own counterintuitive (and sometimes shocking) research findings to show how we're not as rational as we think when we make decisions.

Science educator Arvind Gupta uses simple toys to teach.

Safwat Saleem uses satire and art to bring to light stories of adversity.

Gonzalo Vilariño uses sport to change the rules of the game for the disabled.

Erik Johansson creates realistic photos of impossible scenes -- capturing ideas, not moments. In this witty how-to, the Photoshop wizard describes the principles he uses to make these fantastical scenarios come to life, while keeping them visually plausible.

In a timely talk, cultural critic Virginia Postrel muses on the true meaning, and the powerful uses, of glamour -- which she defines as any calculated, carefully polished image designed to impress and persuade.

Talking at the US State Department this summer, Hans Rosling uses his fascinating data-bubble software to burst myths about the developing world. Look for new analysis on China and the post-bailout world, mixed with classic data shows.

Accepting his 2007 TED Prize, war photographer James Nachtwey shows his life's work and asks TED to help him continue telling the story with innovative, exciting uses of news photography in the digital era.

Researcher Hans Rosling uses his cool data tools to show how countries are pulling themselves out of poverty. He demos Dollar Street, comparing households of varying income levels worldwide. Then he does something really amazing.

In this fun, 3-min performance from the World Science Festival, musician Bobby McFerrin uses the pentatonic scale to reveal one surprising result of the way our brains are wired.

Golan Levin, an artist and engineer, uses modern tools -- robotics, new software, cognitive research -- to make artworks that surprise and delight. Watch as sounds become shapes, bodies create paintings, and a curious eye looks back at the curious viewer.

At 18, Natalie Warne's work with the Invisible Children movement made her a hero for young activists. She uses her inspiring story to remind us that no one is too young to change the world.

It may seem that big problems require big solutions, but ad man Rory Sutherland says many flashy, expensive fixes are just obscuring better, simpler answers. To illustrate, he uses behavioral economics and hilarious examples.

About this ad: This adorably quirky ad uses sleight of hand to celebrate the curiosity of Honda engineers, highlighting some of their most successful innovations over the past 65 years.

I wish for us to discover the millions of unknown archaeological sites across the globe. By building an online citizen science platform and training a 21st century army of global explorers, we'll find and protect the world's hidden heritage, which contains clues to humankind's collective resilience and creativity.

Imagine having a surgery with no knives involved. At TEDMED, Yoav Medan shares a technique that uses MRI to find trouble spots and focused ultrasound to treat such issues as brain lesions, uterine fibroids and several kinds of cancerous growths.

Artist Uldus Bakhtiozina uses photographs to poke fun at societal norms in her native Russia. A glimpse into Russian youth culture and a short, fun reminder not to take ourselves too seriously.

Bruce Bueno de Mesquita uses mathematical analysis to predict (very often correctly) such messy human events as war, political power shifts, Intifada ... After a crisp explanation of how he does it, he offers three predictions on the future of Iran.

"An advanced city is not one where even the poor use cars, but rather one where even the rich use public transport," argues Enrique Peñalosa. In this spirited talk, the mayor of Bogotá shares some of the tactics he used to change the transportation dynamic in the Colombian capital... and suggests ways to think about building smart cities of the ...

Del Harvey heads up Twitter’s Trust and Safety Team, and she thinks all day about how to prevent worst-case scenarios -- abuse, trolling, stalking -- while giving voice to people around the globe. With deadpan humor, she offers a window into how she works to keep 240 million users safe.

Vikram Sharma uses quantum technology to strengthen cryptographic key management and encryption.