https://www.youtube.com/watch?v=Uqno63OiLbI

1. duolingo competes with candy crush, so they made duolingo like a came
2. north star - DAU - want users to return to app everyday as learning a language requires daily practice
3. badges and streaks worked the best for them
4. relied to ab testing heavily, optimized everything from messaging, notifications, etc.
5. PR worked out for them in giving a usage spike, which then retained.