The creator of Duolingo was also the creator of ReCaptcha I think. He has a really interesting ted talk about it here: https://www.ted.com/talks/luis_von_ahn_ma

The creator of Duolingo was also the creator of ReCaptcha I think. He has a really interesting ted talk about it here: https://www.ted.com/talks/luis_von_ahn_massive_scale_online_collaboration https://www.ted.com/talks/luis_von_ahn_massive_scale_online_...