“You don't find out whether you understand something by reading it again. You find out by trying to say it.”
You finish a chapter, you close it, and you think "yeah, I get that". Two days later someone asks you about it and you produce about four words and then nothing.
That gap is the most frustrating thing about studying, and for a long time I thought it meant I had a bad memory. It doesn't. It means the feeling of understanding turned up before the understanding did, and I believed it.
The feeling arrives first, and it lies
When you read a good explanation, it's clear. Obviously it's clear, someone spent a long time making it clear. The problem is that the clarity belongs to them, and while you're reading it, it feels like it belongs to you.
That's the whole trap. You're not measuring whether you can explain this. You're measuring how smooth it was to read. Those two things feel identical from the inside and they're completely unrelated.
Which is why "I've read it three times" tells you nothing, and why people walk into exams genuinely surprised.
Four tests, easiest to hardest
You don't need all four. Use the first one that makes you stall.
1. Say it out loud with everything closed. Not in your head. In your head you skip the bits you don't have, and you don't notice you skipped them. Out loud you hear yourself stop.
2. Explain it to someone who doesn't do your subject. This is the one that catches most people. You're not allowed the technical words, so you have to actually know what they stand for. If you can't say it without the jargon, the jargon might be doing all the work.
3. Answer one more "why". You said osmosis is water moving across a membrane. Fine. Why does it move? And why that direction? Most understanding is one layer deep and looks fine until someone asks a second question.
4. Use it on something you haven't seen. A different example, a case from a different chapter, a made-up situation. This is what an exam does, and it's the only test of the four that can't be passed by memory.
The stall is the point, not the failure
The moment you stop halfway through a sentence feels bad. It feels like proof you're behind.
It's the opposite. That's the first honest information you've had all day. Every minute before it you were reading things you already recognised and feeling fine. The stall is the tool doing its job.
What matters is what you do in the ten seconds after it. Most people go back and reread the whole section, which mostly restores the nice feeling without fixing anything. Instead, write down the exact sentence you couldn't finish. That sentence is your actual gap, and it's usually much smaller and more specific than "I don't get this chapter".
Jargon is where not-understanding hides
If your explanation is "active transport uses ATP to move solutes against the concentration gradient", you might understand it perfectly. Or you might have memorised a sentence.
There's an easy way to tell. Try saying it with none of those words. If what comes out is "it spends energy to push stuff the way it doesn't want to go", you're fine. If nothing comes out, the technical words were holding the whole thing up.
This isn't an argument against technical language. You need it, and you'll be marked on using it. It's just that jargon is very good at hiding a hole, including from you.
Explaining it simply without making it wrong
The advice everyone gives is "explain it like they're five". It's decent advice with one failure mode: you can simplify something until it's comfortable and false, and then feel great about it.
The test isn't whether it's simple. It's whether someone could act on it. If your simple version would let a person predict what happens next in a situation, it's a real explanation. If it just sounds nice, it's a slogan.
The awkward part: doing this alone is hard
Everything above works better with another person, and that's the practical problem. Your friends are busy, your flatmate doesn't care about osmosis, and explaining to a mirror only catches the biggest gaps.
The mirror doesn't ask a follow-up. It never says "okay, but why that direction?" And the follow-up is where the second layer of understanding gets tested, which is test three on the list, which is where most people are actually stuck.
That's the specific reason Leurons has a tutor that makes you type an explanation and then pushes back on it, rather than just giving you the answer. It's the one part of studying that genuinely needs someone on the other side, and it's the part that's hardest to get at midnight.
The short version
Reading it again tells you nothing. Close everything, say it out loud, drop the technical words, and answer one more "why".
When you stall, don't go back to the start. Write down the sentence you couldn't finish. That's the thing you actually have to learn, and it's usually one small thing rather than the whole chapter.