If you cannot explain it simply you don't understand it well enough
Most of a current education is about teaching abstract theory with way too little integration with reality. The weight of learning should be more placed in using the skills in real-life scenarios. In soccer there is this distinction on The process of learning is learning theory and integrating it in reality. The integration is a crucial part because this is how knowledge can be applied. This allows student to materialize learning and therefore make him remember the abstract knowledge. This gives energy for future learning because it is empowering to be able to apply the knowledge in life.
Example: Anna is learning theory of chemistry of water but she didn't integrated her knowledge and left didn't let water out of radiators in her mountain house for the winter. Anna radiators got smashed – it is an exception in the chemical world – water expands in a frozen state.
On the top there is a practical knowledge, on the bottom theoretical. The more you try to learn about any subject the more there is to learn. The deeper one gets into details of theory the more other fragments one need to uncover. The more foundational you try to understand some area the more knowledge from adjacent disciplines one need.
Anna's knowledge is represented by blue dots. She know some theory here and there. But her dot's doesn't connect and isn't integrated with reality. Feynman dot's are green. He covered all the cases by integrating it with reality. By thinking through multitude of scenarios through a common sense. Also he just covered a lot of ground by dedicating tons of time to this problem.
He embodies his learnings in a video where he talks about advanced concepts of chemistry and physics using the the most common sense imagination and the simplest language:
Taking a break is a strategy to solve puzzles. Leonardo da Vinci talked about importance of taking break. It resets your mind.