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Khan Academy began in 2006 as a tidy collection of math and science tutorial videos from its founder Salman Khan, a hedge-fund analyst who had posted the material to help friends and relatives. Its popularity snowballed, and in recent years Khan has beefed up the software that sits behind and around the video lessons, delivering interactive practice exercises and enabling conversation and remote teaching. From just two programmers at the beginning of 2011 and five programmers a year ago, Khan is up to 20 coders now, not counting 15 summer interns. It could use more engineers, but has deliberately slowed its hiring to preserve its laid-back workplace culture, which includes group outings for bowling, movies, and board games.