1. Comprehensible input
The idea that you learn a language by being exposed to content that's understandable but slightly above your current level comes from the comprehensible input hypothesis. That's the core principle behind Lira: reading real texts instead of textbook sentences, and removing the vocabulary blocker right when it happens.
Krashen, S. (1985). The Input Hypothesis: Issues and Implications.
2. How much vocabulary you need
Linguist Paul Nation quantified the lexical coverage needed to read without constant decoding effort: roughly 90-95% known words for reading to stay productive, 98% to feel nearly effortless. Below 90%, most learners report frustration rather than progress — which is why Lira points most people toward A2/B1 as a starting level.
Nation, P. (2006). How large a vocabulary is needed for reading and listening? Canadian Modern Language Review, 63(1), 59–82.
3. Reviewing at the right time
Every word you translate while reading gets queued into a review schedule managed by FSRS, an open-source algorithm that calculates the optimal moment for the next review, right before the word is likely forgotten. Not a fixed interval, actual published research.
Ye, J., Su, J., & Cao, Y. (2022). A Stochastic Shortest Path Algorithm for Optimizing Spaced Repetition Scheduling. Proceedings of the 28th ACM SIGKDD Conference on Knowledge Discovery and Data Mining, 4381–4390.
How FSRS works in Lira →The science
4. Active recall over re-reading
Lira's crossword puzzles aren't just a side game: reconstructing a word from a clue requires active recall, more demanding than simple recognition. Cognitive psychology research shows active recall anchors memory more durably than passive re-reading.
Roediger, H. L., & Karpicke, J. D. (2006). Test-Enhanced Learning: Taking Memory Tests Improves Long-Term Retention. Psychological Science, 17(3), 249–255.
None of these four mechanisms is unique to Lira. What the app does is chain them with no friction: you read a text you chose, translate a word with a tap, it joins an FSRS review queue, and you work it actively through crossword puzzles.