"If someone knows how to say those 3,000 things, they can already express a lot of things in that language ... although obviously not enough to write an article for CNN," jokes von Ahn.
It seems quite simple at first glance, but the company estimates that hundreds of hours are required to finish any one of their courses.
Until now, more than 100,000 people have received the "Golden Owl", which symbolizes that somebody has graduated in one of the languages taught by Duolingo.
For some of its users, this is a dream come true; as user Profesor_Raitao1154 summed up in one of the Duolingo forum threads:
"I am crying because this site is what I always dreamed of: language learning for free, in the most fun way possible."