In this video segment adapted from NOVA scienceNOW, meet Luis von Ahn, a computer scientist and a professor at Carnegie Mellon who was already at the top of his field at age thirty. Learn about one of his most successful ideas—CAPTCHA—a test that humans can pass but computers cannot, which has been used to improve the security of Internet sites. Explore how he comes up with his innovative ideas, and how CAPTCHAs were reinvented to help digitize old books.