When you see a text a second or third time you recognise it, and that feeling of recognition is easily mistaken for actual understanding. This is the fluency illusion: the ease of processing something gets read as a measure of how well you know it. Rereading and highlighting raise how easy something feels right now (retrieval strength), but they barely build the deeper storage that remains once the text is no longer in front of you (storage strength). Take the source away, and the illusion of mastery collapses.
Passive techniques like highlighting and rereading feel productive precisely because they ask for almost no cognitive effort. You register that something matters, but that is not the same as integrating it into what you already know: explaining why it matters, saying it again in your own words, or finding it later without the text. Without that step you stay at the level of collecting information instead of building knowledge you can apply yourself.
The fix is not a better way of highlighting, but building in friction: forcing yourself to retrieve, explain or generate something without the source at hand. That feels harder and less successful in the moment, precisely because you can fail at it, and that friction is what leaves a lasting trace. The same illusion is not limited to studying: nodding along through a text or a codebase because everything feels familiar does not give you the ability to solve the problem yourself either.
