If you get asked to do something that would take place down the road (say you get asked to speak at a conference that's a year away) and you wouldn't want to do it if it were taking place next week, then don't do it. This advice has helped me evaluate the opportunities that I truly want to dedicate my time to and those that I don't.

I watch a lot of TV. That's how I spend most of my time outside of work. If I had more time, I would fill it 100 percent with watching TV.

The question that motivates my research is, if we can put a man on the Moon with 100,000 [people], what can we do with 100 million?

This sounds strange to many people, but I work out for exactly 16 minutes every day, and do so at maximum speed by running 2.5 miles. This keeps me healthy without taking up too much time so I can stay focused on other things. It's one of the most efficient things I do each day.

I finished reading High Output Management by Andrew Grove, which had such valuable insights for leaders that I've been forcing managers at Duolingo to read the book.

I worked on a number of projects that were used by millions of people and that also served as inspiration to what we now call "crowd-sourcing."

The current business model for language education is the student pays — in particular, the student pays Rosetta Stone $500. The problem with this business model is that 95 percent of the world’s population doesn’t have $500.