Millions of students use Khan Academy’s online videos and problem sets to supplement their schoolwork. Three years ago, Sal Khan and I spoke about developing a tool like the Illustrated Primer from Neal Stephenson’s 1995 novel “The Diamond Age: Or, a Young Lady’s Illustrated Primer.” It’s an education tablet, in the author’s words, in which “the pictures moved, and you could ask them questions and get answers.” Adaptive, intuitive, personalized, self-paced—nothing like today’s education. But it’s science-fiction.