Khan Academy is an educational website that, as its tagline puts it, aims to let anyone “learn almost anything—for free.” Students, or anyone interested enough to surf by, can watch some 2,400 videos in which the site’s founder, Salman Khan, chattily discusses principles of math, science, and economics (with a smattering of social science topics thrown in).

The videos are decidedly lo-fi, even crude: Generally seven to 14 minutes long, they consist of a voice-over by Khan describing a mathematical concept or explaining how to solve a problem while his hand-scribbled formulas and diagrams appear onscreen. Like the Wizard of Oz, Khan never steps from behind the curtain to appear in a video himself; it’s just Khan’s voice and some scrawly equations. In addition to these videos, the website offers software that generates practice problems and rewards good performance with videogame-like badges - for answering a “streak” of questions correctly, say, or mastering a series of algebra levels.