How to attack a problem you have never seen. The frame, the moves that get you started, and the question no other chapter asks: which move applies here?
Free to start · adaptive placement finds your level · reviews timed to your own forgetting.
Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Thinking exactly where it's useful.