Guy Raz interviews the world’s best-known entrepreneurs to learn how they built their iconic brands. In each episode, founders reveal deep, intimate moments of doubt and failure, and share insights on their eventual success. How I Built This is a master-class on innovation, creativity, leadership and how to navigate challenges of all kinds. 
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In 2000 Luis von Ahn was starting his PhD in computer science when he attended a talk and happened to learn about one of Yahoo's biggest problems: automated bots were signing up for millions of free Yahoo email accounts and generating tons of spam. Luis' idea to solve this problem became CAPTCHA the squiggly letters we type into a website to prove we're human. He gave away that idea for free but years later that same idea had evolved into a new way to monetize language learning on the web 

reCAPTCHA and Duolingo: Luis von Ahn

How I built this