https://techcrunch.com/2021/05/03/duolingo-ec1-origin/

Luis von Ahn, an entrepreneur who has dedicated his career to scaling free education, has probably annoyed you more than once. In fact, you’ve likely been annoyed by his work dozens and maybe hundreds of times over the years.

A decade before he co-founded the whimsical and language-learning app Duolingo, one of the most popular education apps in the world with over 500 million downloads and 40 million active users, he was building the technology that would become CAPTCHA, those human-annoying but bot-preventing little tests that pop up when registering or logging in to popular internet services like email.