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The Feynman Booth
For everyone who's ever thought: “I know the words but I can't actually explain any of it.”
Self-explanation and the protégé effect (Chi et al. 1994; Fiorella & Mayer 2013).
Pick a concept and teach it back — out loud in your head, in writing here. If you can't say it simply, you've found the gap. That's the point.