From Michael Nielsen, Using spaced repetition systems to see through a piece of mathematics - Michael Nielsen

I also impose on myself a set of aspirational goals, all variations on the idea of distilling the entire proof to one question and (simple) answer. The aim is to fill in the answers to questions having forms like:

Q: In one sentence, what is the core reason a (complex) normal matrix is diagonalizable?

And:

Q: What is a simple visual representation of the proof that (complex) normal matrices are diagonalizable?

I think of these question templates as boundary conditions or forcing functions. They’re things to aim for, and I try to write questions that will help me move toward answers. That starts with grazing, but over time moves to more structural questions about the proof, and about how elements fit together.