What makes Duolingo and Re-Captcha so amazing

So you probably know Duolingo - website where you can learn foreign languages for free. But did you know that by using Duolingo you`re helping to translate the internet? Well, it amazed me... :D

And I`m sure you know Captcha - the four random letters you have to write to verify yourself on webpages every now and then? You might have heard of Re-Captcha as well - the newer version with two random words you have to copy. But did you know that by writing those two words you`re helping to digitalize books?

Really a great idea - I hope there will be more of such projects!  :)