‘If you can’t program it, you haven’t understood it’.
This was a quote by David Deutsch in his book the beginning of infinity. I prefer a milder version though – ‘If you can’t teach it, you haven’t understood it.’
Once something ‘makes sense’ to us, our mind tricks us into believing that we understand it. Yet, several things make sense to us, but we are unable to teach them to somebody else. Do we really understand those things?
To know an explanation is just the first step. To teach it, we need to commit it to memory, see other places where the principle applies and be able to explain it to somebody else who doesn’t know what we know. If your explanation makes sense to them, you have truly understood it.
If you can’t teach it, you haven’t understood it.