CaptchaA CAPTCHA is a program that can tell whether its user is a human or a computer. You've probably seen them — colorful images with distorted text at the bottom of Web registration forms.

CAPTCHAs are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHAs.

About 200 million CAPTCHAs are solved by humans around the world every day. In each case, roughly ten seconds of human time are being spent. Individually, that's not a lot of time, but in aggregate these little puzzles consume more than 150,000 hours of work each day. What if we could make positive use of this human effort? reCAPTCHA does exactly that by channeling the effort spent solving CAPTCHAs online into "reading" books. 

After re-purposing CAPTCHA so each human-typed response helps digitize books, Luis von Ahn wondered how else to use small contributions by many on the Internet for greater good. At TEDxCMU, he shares how his ambitious new project, Duolingo, will help millions learn a new language while translating the Web quickly and accurately — all for free.