How Duolingo's AI Learns what you Need to Learn: The language-learning app tries to emulate a great human tutor

It's Lunchtime when your phone pings you with a green owl who cheerily reminds you to “Keep Duo Happy!” It's a nudge from Duolingo, the popular language-learning app, whose algorithms know you're most likely to do your 5 minutes of Spanish practice at this time of day. The app chooses its notification words based on what has worked for you in the past and the specifics of your recent achievements, adding a dash of attention-catching novelty. When you open the app, the lesson that's queued up is calibrated for your skill level, and it includes a review of some words and concepts you flubbed during your last session.