What unique product feature did Duolingo introduce in 2020 that dramatically increased user retention by incorporating linguistic research on memory formation?

Language learning apps have revolutionized how we approach linguistic education, with innovative features constantly emerging to enhance user experience and effectiveness. Behind these advancements are fascinating product decisions that balance technological capabilities with consumer psychology. Test your knowledge about one of the industry's most recognized language learning platforms and its product strategy.

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Stories feature with contextual learning scenarios
Spaced repetition algorithm (Hearts system) that adjusts review intervals based on individual error patterns
Character-based animations that respond to correct/incorrect answers
Audio-only exercises designed for commuter learning