Margen
In 2002, two Yale psychologists, Leonid Rozenblit and Frank Keil, asked people to rate how well they understood everyday objects: a zipper, a toilet tank, a bicycle. High marks, total confidence. Then came the hard part: explain it step by step, as if to a child. The explanations crumbled, and people's self-ratings collapsed with them. They called it the «illusion of explanatory depth»: we mistake being familiar with something for understanding it.
Familiarity disguises itself as knowledge. You've seen a thousand zippers, therefore you «know» how they work. Until you try to spell it out and discover you only had the label, not the mechanism.
Out of this comes a wonderfully cheap tool: before considering a topic learned — for an exam, a decision, an argument — explain it out loud without looking anything up. Wherever you get stuck, that's precisely what you're missing. It's not a shame test: it's a map of gaps.
It even works on opinions: in a later study, asking people to explain in detail how the policy they passionately defended would actually work made them, along the way, a little more moderate.