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Does this feel familiar? 1. You read a book and feel you understood it. 2. Next week, you try to answer a question about it. 3. Surprise: you don't remember the details at all! It's not just embarrassing at a cocktail party: it's a real barrier to learning complex new topics.

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Happily, there's a way to remember anything almost effortlessly: spaced repetition. A few minutes each day—as much memory as you want. Magic. But few people use it. Textbooks don't leverage it. Tools for it desperately lack design. What if we fix it up and build it into a book?

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One reason books are hard is: readers run all the feedback loops. "Did I really get that? Should I flip back?" This demands attention to both the content and also the meta. We interleave prose and lightweight spaced repetition to take some of that work off the reader's plate.

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Imagine you’ve just finished a tough section. We say: catch your breath; let’s take stock. We quickly flick through what we learned. Got it all? Great, we’ll review again later, when you've probably forgotten. Something slipped your mind? OK: we’ll review that bit sooner.

6:13 PM · Mar 19, 2019Twitter Web Client

Something felt totally mysterious? You just learned that you left the stove on before you left the driveway. It doesn’t feel so onerous to read a section again on the spot. It would feel much more tiresome to realize that a few days later… so you might not reread the section.

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One reader said: "There was a section where I tanked them all. I thought: wow, I clearly didn’t get whatever I was supposed to get. […] So then I went back and asked ‘why am I not retaining this?’ I’ve never had that with a textbook before: usually the problems are at the end."

That's the retrospective effect. The *prospective* effect is a feeling of safety. Once you’ve read a few sections, now you read each new passage knowing that you’ll have this quick check at the end. It's a save point every few minutes. You're not going to miss anything.

Then, the next day, you'll get your invitation to your first review session. A few minutes of study, repeated over about 20 sessions over the next year, and you'll remember everything forever. It feels like cheating. One reader: "The bang for the buck is extraordinary."