Feynman’s technique for efficient and effective study
Essentially, he would get an empty notebook that would be dedicated to whatever he wanted to learn, and he would fill it with explanations of that, written in simple concrete language, as though teaching it to someone with no background in the field.
By doing this, it quickly became apparent when he entered an area he didn’t really know (though he may have thougtht he did) because he would find himself unable to explain it in simple terms, and unable to answer the “Why?” questions that the person to whom he was explaining it would ask.
This video is from an Open Culture post that’s worth reading, but don’t watch the video there. It has a long and obnoxious advertisement at the beginning.