What should a community do with its unused land? Plant food, of course. With energy and humor, Pam Warhurst tells at the TEDSalon the story of how she and a growing team of volunteers came together to turn plots of unused land into communal vegetable gardens, and to change the narrative of food in their community.

Daphne Koller is enticing top universities to put their most intriguing courses online for free -- not just as a service, but as a way to research how people learn. With Coursera (cofounded by Andrew Ng), each keystroke, quiz, peer-to-peer discussion and self-graded assignment builds an unprecedented pool of data on how knowledge is processed.

A whirlwind of energy and ideas, Stephen Ritz is a teacher in New York's tough South Bronx, where he and his kids grow lush gardens for food, greenery -- and jobs. Just try to keep up with this New York treasure as he spins through the many, many ways there are to grow hope in a neighborhood many have written off, or in your own.

Bees have been rapidly and mysteriously disappearing from rural areas, with grave implications for agriculture. But bees seem to flourish in urban environments -- and cities need their help, too. Noah Wilson-Rich suggests that urban beekeeping might play a role in revitalizing both a city and a species.

Prompted by the Encyclopaedia Britannica ending its print publication, performance poet Rives resurrects a game from his childhood. Speaking at the TEDxSummit in Doha, Rives takes us on a charming tour through random (and less random) bits of human knowledge: from Chimborazo, the farthest point from the center of the Earth, to Ham the Astrochimp, the first chimpanzee in outer space.

In the fall of 2011 Peter Norvig taught a class with Sebastian Thrun on artificial intelligence at Stanford attended by 175 students in situ -- and over 100,000 via an interactive webcast. He shares what he learned about teaching to a global classroom.

Combining projection mapping and a pop-up book, Marco Tempest tells the visually arresting story of Nikola Tesla -- called "the greatest geek who ever lived" -- from his triumphant invention of alternating current to his penniless last days.

Mit einer Kombination aus Projektionen und einem Aufklappbuch erzählt Marco Tempest die visuell berückende Geschichte von Nikola Tesla – auch "der größte Geek, der jemals lebte" genannt – von seiner Erfolgsgeschichte des Wechselstroms bis zu seinen bettelarmen letzten Tagen.

In this short talk, TED Fellow Sarah Parcak introduces the field of "space archaeology" -- using satellite images to search for clues to the lost sites of past civilizations.

Why is 'x' the symbol for an unknown? In this short and funny talk, Terry Moore gives the surprising answer.