In 2004, Salman Khan uploaded a video onto the internet. It was a tutorial in maths for the benefit of his 13-year-old cousin, Nadia. Little did Khan know he would become an online sensation.
Seeing how his YouTube tutorials helped his relatives and friends, Khan began posting more and more new ones in 2006. Students from the US and elsewhere started flocking to his videos.
In 2009, Khan left his job as a hedge fund analyst and founded the Khan Academy, an NGO dedicated to providing a quality education to anyone anywhere - for free.
'I teach the way that I wish I was taught,' says Khan, 36, who grew up in New Orleans, in the US, and now lives with his wife and two children in California. He graduated from the Massachusetts Institute of Technology in mathematics and computer science and also has an MBA from Harvard University.
'The lectures are coming from me, an actual human being who is fascinated by the world around him,' he adds.
The videos are short - they last from seven to 15 minutes. In voiceovers, he explains concepts and problem-solving techniques. Sometimes, he scribbles on a notepad, which appears on the screen, or uses diagrams as teaching aids. To date, Khan has made more than 3,100 videos for his website about a range of subjects from science to economics.
His website also hosts practice tools and rewards users with videogame-like awards for their performance. Users can track their progress in various subjects and measure themselves against others.