Feynman

This walks you through the classic Feynman learning technique: explain a concept in plain language, spot where you're hand-waving or using jargon you can't define, research those gaps, then rewrite it better. It's structured as a four-step template with tables for tracking vague spots and undefined terms. Good for when you're learning something technical and want to actually understand it rather than just memorize definitions, or when you're preparing to teach and need to find the holes in your mental model. The "explain to a 12-year-old" bar forces you to confront what you don't really get yet, which is the whole point.