Imagine a good free program to learn a language. Imagine that this good program is free because when you are using it you are working for them: they use the answers of your exercises to translate the web. That's Duolingo, one of the best web pages to learn a language. You can use Duolingo in your browser or in your smartphone, because there are versions for iPhone and Android. So let's recommend Duolingo to our students!
In this TED talk the founder of Duolingo, a mathematician from Guatemala called Luis von Ahn, tells us about how he invented Duolingo and Captcha:
