The following script is from "Teacher to the World" which aired on March 11, 2012, and was rebroadcast on Sept. 1, 2013. Sanjay Gupta is the correspondent. Denise Schrier Cetta, producer.
Take a moment and remember your favorite teacher - now imagine that teacher could reach, not 30 kids in a classroom, but millions of students all over the world. As we first reported last year, that's exactly what Sal Khan is doing on his website Khan Academy. With its digital lessons and simple exercises, he's determined to transform how we learn at every level. One of his most famous pupils, Bill Gates, says Khan -- this "teacher to the world," is giving us all a glimpse of the future of education.
36-year-old Sal Khan may look like a bicycle messenger, but with three degrees from MIT and an MBA from Harvard, his errand is intensely intellectual. In his tiny office above a tea shop in Silicon Valley, he settles in to do what he's done thousands of times before.
[Sal Khan: We've talked a lot now about the demand curve and consumer surplus. Now let's think about the supply curve.]
He's recording a 10-minute economics lesson. It's so simple - all you hear is his voice and all you see is his colorful sketches on a digital blackboard.
[Khan: In this video we are going to talk about the law of demand.]
When Khan finishes the lecture, he uploads it to his website - where it joins the more than 3,000 other lessons he's done. In just a couple of years he's gone from having a few hundred pupils to more than six million every month.
Sanjay Gupta: Has it sunk in to you that you are probably the most watched teacher in the world now?
Khan: I, you know, I try not to say things like that to myself. You don't want to think about it too much because it can I think paralyze you a little bit.
[Khan: So if we get rid of the percent sign, we move the decimal over...]
He's amassed a library of math lectures...
[Khan: 12 plus four is sixteen...]
Starting with basic addition and building all the way through advanced calculus.
[Khan: We are taking limited delta x approach to zero. It's the exact same thing.]
But he's not just a math wiz, he has this uncanny ability to break down even the most complicated subjects, including physics, biology, astronomy, history, medicine.
Gupta: How much reading do you do ahead of time?
Khan: It depends what I'm doing. If I'm doing something that I haven't visited for a long time, you know, since high school I'll go buy five textbooks in it. And I'll try to read every textbook. I'll read whatever I can find on the Internet.
[Khan: Let's talk about one of the most important biological processes...]