Teach it back

L1 gave the test: explain it from memory and notice where you stall. This lesson turns that into a technique with a mechanism attached.

Why teaching finds what reading hides

Reading an explanation requires only that you follow the connections somebody else made.

Producing one requires you to make them yourself — decide the order, choose the words, bridge each step to the next. Those bridges are where understanding actually lives, and their absence is invisible until you try to build them.

This is why the technique is old and reliable, and it is what people mean by the Feynman technique: explain it plainly enough for a beginner, and every place you reach for jargon is a place you do not understand.

The obstacle has always been finding a patient beginner. That is what changed.

Set it up as a student

The prompt shape that makes this work:

You are a curious student who knows nothing about [topic] but is smart and asks good questions.

I am going to explain it to you. Interrupt whenever something does not follow, when I use a term I have not defined, or when I say something that raises an obvious question.

Do not explain anything to me. Do not correct me yet. Just be confused where a beginner would be confused.

Three prohibitions, and all three are necessary. Left to itself, an assistant will helpfully finish your explanation, correct you mid-flow, or agree encouragingly — and any of those collapses the exercise.

Then explain, badly

Explain out loud or in writing. Do not prepare. The rough version is the point, because you are diagnosing rather than performing.

Watch for:

Questions you cannot answer. The main signal. Precisely located gaps.

Places you reach for jargon. Using the term instead of explaining the idea is how you skip over a thing you do not have.

“It just does”. You have hit the edge.

Explanations that get longer as you go. Usually you are circling something you cannot state.

Then ask for the marking

Only afterwards, switch its role:

Now stop being the student. What did I get wrong, what did I miss, and where was I vague in a way that suggests I do not understand it?

Vague in a way that suggests is the useful clause. Errors are easy to spot yourself later; vagueness that concealed a gap is exactly what you cannot see, because you produced it.

The harder variant

When you can do the above comfortably:

Now be a student who asks awkward questions. Push on anything I glossed over, and ask “but why?” until I cannot answer.

Three or four “but why”s reliably reach the bottom of anybody’s understanding. That is not failure — knowing where your foundation ends is the most useful map you can have, and it tells you what to study next far better than any syllabus.

Where this beats every other technique

Retrieval (L3) tests whether you can produce a known answer. Teaching tests whether the structure holds — whether the pieces connect, whether you can reorder them, whether you can meet an unexpected question.

Those are different capabilities, and the second is what people mean by understanding. It is also the one that survives being asked about it in a meeting a year later.

Try it now (7 minutes)

Pick something you believe you understand well. Set the assistant up as a student with the three prohibitions.

Explain for five minutes. Count the questions you could not answer cleanly. Anything above zero is worth your next study session.

Check your understanding

1. Teaching exposes gaps that reading does not because: Teaching takes longer Producing an explanation requires making the connections yourself Students ask unpredictable questions

2. Why forbid the assistant from explaining or correcting during the exercise? It would take too long It will otherwise finish your explanation or agree encouragingly, collapsing the exercise Its corrections are unreliable

3. Reaching for jargon while explaining usually signals: Appropriate technical precision A place you are using the term instead of understanding the idea That the student needs more background

Recap

Producing an explanation requires making connections that following one does not, so teaching surfaces gaps immediately. Cast the assistant as a curious beginner and forbid it from explaining, correcting or agreeing. Explain roughly, watch for unanswerable questions, jargon and “it just does”, then switch its role and ask where you were vague in a way that suggests you do not understand.