The so-called flexibility is to draw inferences from other things. How can we be flexible? We should grasp that "one" and really grasp the core of what we have learned, instead of just knowing how to do a few questions. How to grasp the core? There are two standards: 1. Can you say what you have learned in your own language? 2. Can you give others or yourself two questions without reading the information? If you do these two things, you will grasp this "one" and you will be able to "reverse the three" and be flexible.