Week 3 Session: What the closed source reveals
Study a supplied explanation of a pin tumbler lock, close it, rebuild the mechanism from memory, then restore it and annotate the difference by type of loss before defending one judgement from what you produced.
8 March 2027
Purpose and preparation#
This self-guided investigation applies Week 3’s lesson. Allow about 30 minutes, plus 10 more for the partner variant in step 7. Everything you need is on this page. You need something to write with and nothing else: no lock, no tools, no paid book, no AI account. Priya is fictional.
You are examining evidence of understanding, not sitting a test about locks. You will not learn to open one, and nothing here is instruction in picking.
Work alone, or with a partner using the variant in step 7. The reference below is hidden behind a toggle on purpose: opening it is a step in the procedure, and the procedure asks you to close it again.
If you did the slide exercises, reuse those answers and start at step 4.
1. Declare what you already know — 1 minute#
Write one line: what did you already know about how a key lock works, and where from? A locksmith, a disassembled padlock, a video, nothing at all.
Record it before you read anything. Prior familiarity changes what a reconstruction demonstrates, and a line written afterwards is hard to trust — including by you.
2. Study the reference — 4 minutes#
Open the toggle, read it once or twice, then close it and leave it closed until step 5 says otherwise.
The reference: how a pin tumbler lock works
An outer casing has a cylindrical hole in it. Inside that hole sits a rotating cylinder called the plug. To unlock the door, the plug has to turn. The front face of the plug carries a straight slot, the keyway, which the key enters. At the back of the plug is a cam: a projecting piece that, as the plug turns, draws back the bolt, the metal bar holding the door shut.
Five or six holes are drilled down through the plug. Each holds a key pin, cut to its own length and rounded underneath so a key can slide across it. Directly above each key pin, in a shaft in the casing, sits a driver pin, with a spring above it pressing down. A key pin and the driver pin above it make up one stack. The boundary where the plug meets the casing is the shear line: the surface along which the plug has to rotate.
With no key, the springs push each stack down, so every driver pin is forced part-way out of the casing and into the plug. Pins cross the boundary, and the plug cannot turn.
With the correct key, its cut edge lifts each key pin by a different, exact amount, so every key-pin/driver-pin junction comes level with the shear line. Nothing crosses the boundary, the plug turns, and the cam draws the bolt back.
With a wrong key, the cuts lift the stacks by the wrong amounts. Lifted too little, a driver pin still crosses into the plug; lifted too far, a key pin is pushed up into the casing. One wrong stack is enough to hold the plug.
Technical grounding: Wikipedia’s article on the pin tumbler lock, which names the boundary the shear point. That article grounds the mechanism and is not evidence for any claim about understanding.
3. Reconstruct it — 8 minutes#
With the reference closed, write out how a pin tumbler lock works in your own words. Name the parts. Say where each one sits relative to the others. Explain why the plug cannot turn with no key, why it turns with the correct key, and what a wrong key does.
Preserve this text exactly as you first write it. Everything after this step works on it, and a tidied version destroys the record.
About the closed toggle. You can reopen it, and nobody is watching. A source removal you administer to yourself is not something another person can check. Follow the instruction anyway for a practical reason: a copied version leaves step 5 nothing to examine, so you lose the exercise rather than fail a test. If you do look, write “reconstructed with one look at the reference” in your record. A recorded look is usable evidence; an unrecorded one is what ruins the page.
4. Note what it felt like — 1 minute#
Before you check anything, write one line: which part of the mechanism felt least secure while you were writing?
Keep this separate from the annotation in step 5. It is a self-report about the experience of writing, not evidence about the text. Comparing the two later is interesting precisely because they often disagree.
5. Restore the source and annotate — 5 minutes#
Reopen the toggle in step 2 and read it beside your own version. Do not rewrite your text. Mark each difference in the margin with one letter:
| Letter | What it marks | Example of the loss |
|---|---|---|
| P | A part omitted altogether. | No springs anywhere in your text. |
| R | A relation omitted: parts named but not placed. | Driver pins named, but not said to sit above the key pins in the casing. |
| C | A causal step omitted: a “because” the mechanism needs. | The plug turns with the right key, but nothing says why a clear boundary allows it. |
| X | A scenario omitted: a case the reference covered. | No account of over-lifting by a wrong key. |
Then count them: how many P, R, C and X.
Compare with a fictional annotated reconstruction
Priya wrote:
“A door lock has a barrel that turns when you put the right key in. Inside there are pins of different heights. The key pushes the pins up to the right heights, and then the barrel can turn and the door opens. A wrong key does not push them to the right heights, so it will not turn.”
Nothing in it is false. Every sentence is consistent with the reference, which is exactly why counting matters more than judging it by feel.
P — four parts gone: springs; driver pins, since she has one set of pins where the reference had two; the cam; the bolt. R — a barrel with pins “inside,” but no casing and nothing about driver pins sitting above key pins. C — the shear line is gone, so “right heights, so it turns” states a rule with no mechanism under it. X — the wrong key appears, but only as under-lifting.
Counts: P 4, R 2, C 1, X 1.
If your own text contained most of the reference, annotate it that way and say so. A strong performance is a result, not a problem with the exercise.
6. Weigh the annotation — 4 minutes#
A count is not yet a judgement. Write two or three sentences deciding which of your marks matter and why.
How the four types differ as evidence
A missing P is often a vocabulary gap. You may hold the arrangement and lack the phrase “driver pin,” and describing the same arrangement in your own words would show it.
A missing C is the strongest finding of the four. Without the causal step there is no account of why the outcome follows, so the text could have been produced by someone who never took in the boundary at all. Priya can say what happens; she cannot yet say what makes it happen. A lost noun and a lost “because” are not the same loss.
A missing R sits between them: the parts are there, so something was retained, but an unplaced part does no work in the mechanism.
A missing X is the least conclusive. You may have judged the wrong-key case not worth writing rather than not having it. Where it matters, ask for the scenario directly instead of inferring from its absence.
7. Optional: make the removal witnessed — 10 minutes#
With a partner. Give them this page. They read the reference aloud once, then keep it and ask you to reconstruct it while they watch. They annotate your text against the reference and report the counts.
This fixes one specific defect: the closure of the source becomes an observation somebody else can report, rather than your own account of it. That is a genuine improvement in the condition, and it is the only thing it improves.
Working alone, you lose exactly that. Your record still stands, but the line “reference closed” is self-report, and self-report is not independent evidence of how an answer was produced. Write the condition as self-administered and unwitnessed and claim less from the result.
A partner adds observation of the conditions. It does not give either of you access to how the text was actually produced — whether it was rebuilt in the moment or remembered whole from four minutes earlier.
8. Keep the record — 4 minutes#
Preserve this with your annotated page. Week 4 asks you to compare against it.
| Field | What to write |
|---|---|
| Prior familiarity | Your line from step 1, unrevised. |
| Conditions | Reference closed or looked at; alone or witnessed; written or spoken; time taken. |
| Original text | Your reconstruction, preserved before annotation. |
| Counts | P, R, C, X. |
| Felt gap | Your line from step 4, and whether the annotation agreed with it. |
| Observation | What your text actually contained. |
| Interpretation | The specific thing it shows you can produce. |
| Alternative | Could verbatim memory, a look back, or prior familiarity explain it? |
| Limit | The claim that would still need more evidence. |
9. Defend one judgement — 3 minutes#
Finish by writing two sentences about your own page: one claim the annotation supports, and one it does not. Name the alternative explanation you cannot rule out for each.
For example: “With the reference closed on my own account, I produced all three states with the shear line doing the causal work. That supports the claim that I can rebuild this mechanism’s structure minutes after reading it. It does not show that I built the structure rather than remembered the passage, because nobody observed the closure and I had read it four minutes earlier.”
If your reconstruction was complete, take that seriously rather than looking for something to have gone wrong. Say what it establishes — a structure produced with the source out of view, at that moment — and then say that it does not establish that the structure was built rather than memorised, that the source was truly closed, or that the same would happen for a mechanism you had not just read.
Do not conclude that reconstruction is the better way to read. It is a measuring instrument, and you could not have reconstructed anything you had never consumed. Week 4 takes this same lock and changes the conditions around it, which is the only way to find out how much of today’s result belonged to the task rather than to you.
The spec
What you bring to the session. Written so a reader can tell whether it has been met without asking you.
- a reconstruction of the supplied mechanism written with the reference closed
- the reconstruction annotated for omitted parts, relations, causal steps and conditions
- counts of each omission type recorded beside the conditions the reconstruction was produced under
- one claim the annotated page supports and one it does not, each with an alternative explanation it cannot rule out