'There's no reason it shouldn't be 20 million.' In any given month, Khan says, he's reached about 200,000 students. Most page views come from the U.S., followed by Canada, England, Australia, and India. Since he began his tutorials in late 2006, Khan Academy has received 18 million page views worldwide, including from the Gates progeny. Khan's playlist of 1,630 tutorials (at last count) are now seen an average of 70,000 times a day - nearly double the student body at Harvard and Stanford combined.

Khan Academy, with Khan as the only teacher, appears on YouTube and elsewhere and is by any measure the most popular educational site on the web. It is here that Salman Khan produces online lessons on math, science, and a range of other subjects that have made him a web sensation. In an undistinguished ranch house off the main freeway of Silicon Valley, in a converted walk-in closet filled with a few hundred dollars' worth of video equipment and bookshelves and his toddler's red Elmo underfoot, is the epicenter of the educational earthquake that has captivated Gates and others.

It was a good day his wife let him quit his job.' Khan wasn't even there - he learned of Gates' praise through a YouTube video. Ruminating on what he called the 'mind-blowing misallocation' of resources away from education, Gates touted the 'unbelievable' 10- to 15-minute Khan Academy tutorials 'I've been using with my kids.' With admiration and surprise, the world's second-richest person noted that Khan 'was a hedge fund guy making lots of money.' Now, Gates said, 'I'd say we've moved about 160 IQ points from the hedge fund category to the teaching-many-people-in-a-leveraged-way category. Then, several weeks ago, at the Aspen Ideas Festival in front of 2,000 people, Gates gave the 33-year-old Khan a shout-out that any entrepreneur would kill for. 'It is awesome how much he has done with very little in the way of resources.' Gates and his 11-year-old son, Rory, began soaking up videos, from algebra to biology. This past spring a colleague at his small think tank, bgC3, e-mailed him about the nonprofit, a vast digital trove of free mini-lectures all narrated by Khan, an ebullient, articulate Harvard MBA and former hedge fund manager. Gates is a voracious consumer of online education. FORTUNE - Sal Khan, you can count Bill Gates as your newest fan.

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