By early 2009, tens of thousands of students were watching tutorials on the Khan Academy every day. The software I wrote for my cousins had become so popular it was making my $50-a-month web host crash. The possibilities surrounding the academy were so exciting that I had trouble doing my day job properly. And soon I quit.
In retrospect I was unbelievably naive. Despite already having more views on YouTube than MIT OpenCourseWare and Stanford combined, the Khan Academy was still a one-person operation run out of a closet. Then I got an unexpected and providential e-mail from Ann Doerr, wife of famed venture capitalist John Doerr. She suggested we have lunch. When Ann asked how I was supporting myself and my family, I answered, “I’m not; we’re living off of savings.” She nodded, and we each went our ways. About 20 minutes later I got a text message as I was parking in my driveway. It was from Ann: You need to support yourself. I am sending a check for $100,000 right now. I almost crashed into the garage door.
Two months later I got another text message from Ann.
At Aspen…hundreds of people in audience…Bill Gates on stage, talking about you.
My mind immediately pictured all the half-assed videos I had made for my cousins — where my son is screaming in the background or I’m trying to cram in a concept before my wife comes home from work. Did Bill Gates really watch those?
After about a week, Gates’ chief of staff called. He told me that Bill would like to fly me to Seattle to meet and see how he could support the Khan Academy. I was staring at my calendar as he was asking my availability; it was completely blank for the next month. Sitting in my closet and trying to sound as cool as possible, I said, “Sure, I think I could squeeze something in.”
MORE: Sal Khan – building a better university
The meeting happened on Aug. 22. I was waiting in a conference room with several other people from the Gates Foundation. They reassured me that “Bill is just another human being. He’s completely cool.” After a few minutes all the people in the room began to look a bit more serious than they had 30 seconds before. Bill Gates had walked in and was standing behind me. Yeah, just another human being. I spent the next 15 minutes talking about what I thought the Khan Academy could do and how we would do it. He asked me a few questions and then said simply, “This is great.” Two days later, an article came out in Fortune magazine titled “Bill Gates’ Favorite Teacher.” The headline was unreal. The article made my mother cry. It seemed that it was time for me to move out of my closet office.
Adapted from The One World Schoolhouse, published this month by Twelve Books
This story is from the October 8, 2012 issue of Fortune.