CAMBRIDGE, Mass. (AP) — Luis von Ahn used to be best known as an inventor of the security tool reCAPTCHA, which helps digitize books every time you type those distorted letters to prove to websites you’re human.

Now the 40-year-old entrepreneur is better known as co-founder and CEO of a popular language-learning app, Duolingo. The mostly ad-funded app works like a video game, motivating users to keep translating words correctly by rewarding them. The Pittsburgh-based startup had more than $40 million in revenue this year and boasts 300 million users worldwide.

Von Ahn spoke with The Associated Press after winning the prestigious Lemelson-MIT Prize for inventors who’ve made a positive social impact. The interview has been edited for length and clarity.

Q: Why did you shift your efforts to Duolingo?

A: I wanted to do something related to education. Where I’m from, Guatemala, people who have a lot of money can buy themselves the best education in the world. People who don’t have money barely learn to read and write. In non-English speaking countries, you can basically double your salary by knowing English. I wanted to give people a free way to learn English.

Q: How are the reCAPTCHA and Duolingo business models alike?

A: At first, the way we wanted to make Duolingo free is to have people translate stuff without knowing. It turned out it was much better to put ads.

Q: Are you surprised by the customers you have?

A: I thought most of the people using Duolingo would actually be interested in learning a language. Many are, particularly for learning English. But in countries like the U.S., where learning a language is more of a hobby, a ton of our users are people whose interest is doing something productive with their time. Their primary goal is to sort of be entertained, and not feel guilty about it.

Q: Who do you see as your biggest competitors?

A: We’re competing for time. It’s very rare that people leave Duolingo for another language-learning method. Usually people leave Duolingo for Instagram or “Candy Crush.”

Q: Like working out, learning a language requires a time commitment. How do you keep people interested?

A: We do have people we’ve hired, mainly from gaming companies. It’s what sets Duolingo apart from other learning platforms. We have really spent a lot of effort trying to keep people addicted to Duolingo.

Q: Isn’t addiction a bad thing?

A: What’s going on today is a lot of companies, gaming companies, Facebook, are using data from millions of people to figure out how to get you addicted. They’re using sophisticated artificial intelligence to find out the exact right time to send you a notification or surface a post. We’re using the same techniques but we feel good about it because we’re getting you addicted to learning.