TL;DR:
Explain the idea in plain words as if to a 12-year-old, out loud or on paper, with no notes open.
Wherever you stall, hand-wave or reach for jargon, you have found a gap. Go back to the source for exactly that gap.
Then simplify again. A good explanation gets shorter, not longer.
You finish the photosynthesis chapter and feel fine about it. You can recognise every term: chlorophyll, thylakoid, Calvin cycle, ATP. Then a friend asks "so how does a leaf turn light into sugar?" and what comes out is "um, it uses sunlight to make glucose, with, like, chlorophyll."
That stumble is the most useful thing that has happened all evening. The Feynman technique is a way of creating it on purpose, on a night when it costs nothing, so the gaps show up before the test does.
Why explaining shows what reading hides
Reading lets you move along a text at the speed it gives you. Every sentence arrives already connected to the next one, so you can follow it without being able to produce it. Explaining flips that: you have to generate the sentences and the links between them yourself.
Plain words matter because jargon can stand in for understanding. You can say "the light reactions produce ATP" without knowing what that means or why the plant wants it. If you have to say it to someone with no biology, the stand-in word has to be replaced with what it actually does.
What you need
A blank page or an empty text box. No notes, no textbook open.
A listener, real or imagined: a younger sibling, a rubber duck, or just a voice memo. The point is speaking to someone who does not already know.
Ten to twenty minutes, and the chapter within reach for later.
The four passes
Name the topic and write it as if to a 12-year-old: Put the topic at the top and explain it from scratch in short sentences with everyday words.
Find the gaps where the explanation breaks: Mark every place you paused, waved your hand, repeated a term without meaning, or said "it just does". Those marks are your study list.
Go back to the source: Open the book or notes only for the marked gaps. Read just enough to fill each one, then close it again.
Simplify again: Re-explain the whole thing, and try to cut it down. If you can use an analogy that is accurate, keep it. If the analogy makes a false claim, drop it.
Worked example: explaining photosynthesis to a 12-year-old
First attempt, from memory
"Plants take in sunlight and make glucose. They use chlorophyll, which is why they are green. They take in carbon dioxide and give off oxygen. It happens in the chloroplast."
That sounds fine, and it has four gaps. Here they are with what the writer found when they went back to the source.
Where the explanation broke | What the source says |
|---|---|
"Chlorophyll is why they are green" (but why would a light-catcher be green?) | Chlorophyll absorbs mostly red and blue light and reflects green, so green is the light that is not used. |
"Gives off oxygen": from where? | The oxygen released comes from splitting water, not from the carbon dioxide. |
"Makes glucose": out of what, and with what energy? | Carbon from CO₂ is built into sugar in the Calvin cycle, using ATP and NADPH made earlier from light. |
"In the chloroplast": where, exactly? | Light reactions happen in the thylakoid membranes; the Calvin cycle happens in the stroma around them. |
Second attempt
"A leaf is a tiny factory with two rooms. In the first room, green stuff called chlorophyll catches red and blue light and uses that energy to split water. That makes oxygen, which floats out, and it charges up two little batteries, ATP and NADPH. In the second room, the plant takes carbon dioxide from the air and uses the charged batteries to stick the carbon together into sugar."
It is not perfect, but each claim now has a reason behind it. A 12-year-old could ask "why does it split water?" and you have an answer: to get the energy-carrying electrons.
Checking the equation
To check the story against numbers, balance the overall reaction: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon: 6 on each side. Hydrogen: 6 × 2 = 12 on the left, 12 on the right. Oxygen: 12 + 6 = 18 on the left, 6 + 12 = 18 on the right. It balances.
It also raises a question worth chasing. Six O₂ molecules hold 12 oxygen atoms, but 6 water molecules hold only 6. The fuller account is that 12 waters are split and 6 are made again in the Calvin cycle, so the net is 6, and all the released oxygen does come from water. Your textbook may show only the short form; use whichever form your course uses on tests.
Third attempt: simplify
"Plants use light to charge batteries, then spend the batteries to turn air into sugar." If that sentence feels true and you can unpack each half, you are done.
Mistakes that waste the technique
Reciting instead of explaining. If you are repeating the book's sentences, you are testing memory of wording, not understanding.
Using jargon as a shortcut. If the word "photosystem" appears and you cannot say what it does, you found a gap, not an answer.
Opening the book too early. Fill the gap after you have written the first attempt, not during it.
Making the analogy do the work. An analogy that explains everything usually breaks somewhere. Say where.
Your plan for this week
Pick one topic from this week's class that you feel fine about and run all four passes on it, with a timer set for fifteen minutes.
Keep the list of gaps. Re-explain the topic in two days with nothing open, and see which gaps are still there.
Do it once with a person. Let them interrupt with "why?" at least three times.
Frequently asked questions
Do I need to be able to teach it perfectly?
No. The goal is to find out what you cannot yet explain, not to give a flawless lecture.
Can this work for maths?
Yes. Explain why each step of a method is allowed, not just what the steps are. The gaps show up as steps you can do but cannot justify.
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