Week 2 Session: What your explanation leaves out

Run the rate, explain and re-rate protocol on a zip fastener, annotate your own explanation against a technical description, and defend one claim from what you actually produced.

1 March 2027

Purpose and preparation#

This self-guided investigation applies Week 2’s lesson. Allow about 30 minutes, plus 15 more if you complete step 7. Everything you need is on this page. Work alone or with a partner; no paid book, AI account, special equipment or physical zip is required, and you should not look anything up until step 4 tells you to. Nadia and Theo are fictional.

You are examining evidence of understanding, not sitting a test about zips. If you already know how a zip works, say so in your record rather than pretending the material is new. If you did the slide exercises, reuse those answers and start at step 4.

Keep this as practice in judging a performance from evidence produced under stated conditions, qualified by what that evidence cannot establish.

1. Rate yourself — 1 minute#

On a scale of 1 to 7, how well do you understand how a zip works? 1 is the lowest level of understanding and 7 the highest. Rozenblit and Keil, whose protocol this adapts, trained their participants on this scale with a cross bow, which most adults understand mechanistically, and a satellite navigation receiver, which most do not.

Record the number as Rating 1. Do not revise it later; the point of the trail is that earlier entries stay as they were.

2. Explain it — 6 minutes#

Write a detailed, step-by-step causal explanation of how a zip opens and closes. Name the parts and say what each one does to the others. Someone who had never seen a zip should be able to follow you. Do not consult anything.

Preserve your original wording. When you finish, re-rate yourself on the same scale as Rating 2.

3. Answer the diagnostic — 4 minutes#

A diagnostic question is one that cannot be answered without the mechanism itself. Here is ours:

The slider on a zip is a single rigid piece with no moving parts inside it. Pulled one way it closes the zip; pulled the other way it opens it. Explain, step by step, how one rigid object can do both.

Write your answer, then record Rating 3. Attempt this before reading step 4, which contains the description.

4. Read the description — 3 minutes#

Technical background from Wikipedia’s article on the zipper. It grounds the example; it is not evidence for a claim about understanding.

A zip has two tapes, the fabric strips sewn to the garment, each carrying a row of teeth (also called elements) along its inner edge. A slider rides on both rows, moved by a pull tab, with a top stop and bottom stop at the ends.

Inside the slider is a Y-shaped channel. Closing, the two arms of the Y are inlets and the stem is the outlet, so the rows converge and are pressed together one pair at a time. Each tooth has a projection on one face and a recess on the other; the projection is slightly larger than the recess it enters, so the tooth springs apart and closes over its neighbour. That spring is an elastic deformation — the tooth returns to its shape, so the joint holds once the slider has passed, resisting force both across the zip and along it. Opening, the Y is reversed: the stem is the inlet, the arms the outlets, and the wedge between them prises each pair apart in turn.

One boundary condition follows from that geometry rather than from any extra fact: the hold depends on every tooth keeping its shape and having an intact neighbour to grip, so a bent or missing tooth leaves a point where nothing is interlocked and the chain parts there even though the slider has passed.

5. Annotate what you wrote — 4 minutes#

Set your own explanation beside that description. Do not rewrite it. Mark each relation in the description that your text did not contain:

6. Examine the evidence — 5 minutes#

Keep this record. Later weeks will ask you to compare against it.

Two cautions the lesson turns on. First, the rating trail is self-report: a drop tells you your confidence changed, not what you can produce. The independent artefact is the annotated text. Second, running this once on yourself, with no control and no comparison group, is not a replication of Rozenblit and Keil’s study; it is a record of one session.

“I don’t really understand zips” is not a limit. “My text never said what holds two teeth together” is, because it names the missing evidence.

7. Swap, or run the solo route — 15 minutes#

With a partner. One of you names a topic you genuinely know. The other researches it freely for ten minutes, then explains it back with sources closed. The knower probes: the exception, the case where the usual account breaks, the step everyone skips. The probe is not marking; it is finding where the explanation stops being load-bearing. Record what was asked and what the explanation could not reach.

Alone. You can do this later. Repeat steps 1 to 6 on a second everyday mechanism — a door handle, a bicycle brake, a tap — using one freely available technical description at step 4. Then write the single sentence that states the loss: with no second person there is no independent check on whether your explanation actually satisfied someone who knows the material. You grade your text against a description you have just read, so you cannot tell whether you would have produced the missing step unprompted, and every probe you face is one you chose.

8. Finish with the examiner’s sentence#

Write one claim your annotated text supports, and one it does not. For example: “With no sources, I produced a written explanation that named the slider and the teeth but not the mechanism holding them together. This supports the claim that I can describe the visible action of a zip. It does not support the claim that I can explain why the teeth stay joined, and it says nothing about how I would handle a mechanism I had never used.”

That is the form of judgement this course is practising: an observation, a claim no wider than the observation, and a stated limit. Next week removes the source instead of supplying it, and asks what reconstruction reveals that explanation did not.