How Much Can Duolingo Teach Us?

In the fall of 2000, as the first dot-com bubble was bursting, the Guatemalan computer scientist Luis von Ahn attended a talk, at Carnegie Mellon, about ten problems that Yahoo couldn't solve. Von Ahn, who had just begun his Ph.D., liked solving problems. He had planned to study math until he realized that many mathematicians were still toiling away over questions that had proved unanswerable for centuries. "I talked to some computer-science professors and they would say, 'Oh, yeah, I solved an open problem last week,' " he told me recently. "That seemed just a lot more interesting."