let's talk about one of the most important biological processes... >> gupta: sal khan has tackled so that, if you watched just one of his lectures a day, it would take over eight years to cover it all. >> khan: these are huge time scales... magnetic north is kind of the geographical... and let's say this is point x is equal to... basic introduction... light... if this does not blow your mind, you have no emotion. >> gupta: did you ever think about putting yourself visually in the video? >> khan: look, if there's a human face there, especially a funny-looking human face, then it's actually hard to focus on the math. 4,000 is 2,000 x 3 is 6,000. i don't have to shave, i don't have to comb my hair. i just press "record," make a video. there might be spinach in my teeth. who cares? >> gupta: the format is so simple. why does it appeal to so many people? >> khan: i've gotten a lot of feedback that it really does feel like i... i'm sitting next to the person and we're looking at the paper together. let me take out my trusty calculator out. i'm 95% of the time working through that problem re