I think one indicator of whether someone is an expert is the variety of concepts they’ve defined in their own terms.
To organize a concept in your own terms, the following conditions must be met:

• Have you deeply understood that concept?
• Can you connect the understood concept with other concepts you normally have?
• Can you redefine that concept using the connected concepts?

If all four conditions are met, the concept has been newly defined, but it must also be valid to others. A learning technique that fulfills these conditions well would be the Feynman Technique. (In fact, I think explaining a concept to someone is a process of reflecting on whether you truly understand that concept yourself. From my personal experience, whenever I explained a concept I didn’t properly understand, there was always something that felt blocked.)

However, due to laziness, impatience, and other reasons, I had been reluctant to learn this way for several years, and the results over those years were quite poor. So I’ve decided to post writings that redefine even just one or two concepts per week.