Familiarity can masquerade as understanding
A zipper is easy to recognize and use, so your mind rarely needs to build a detailed account of its moving parts. You learn the action—pull the tab, open or close the bag—and the object’s purpose. Each successful use reinforces that practical knowledge, not necessarily a picture of how the mechanism works.
This is a version of the illusion of explanatory depth: people often feel they understand familiar objects better than they do. Recognition is effortless; explanation demands something else. To draw a zipper, you must retrieve details you may never have paid attention to, such as how the slider guides the two rows of teeth and how those teeth meet.
Using it doesn’t require reverse-engineering it
As the slider moves, its shaped channel brings the rows together or pulls them apart. The teeth are arranged to mesh when guided into alignment. Your hand supplies the motion, while the slider does the precise guiding. In everyday use, you can rely on that sequence without knowing its geometry—just as you can ride a bicycle without being able to sketch its chain and gears.
A drawing task changes the demand. Instead of spotting a zipper or operating one, you have to produce its structure from memory. That exposes the difference between knowing what an object does and knowing how its parts cause it to happen. The surprise is useful: it shows why familiarity alone is a poor test of understanding. Try explaining the mechanism step by step, and the missing details become much harder to overlook.
