The so-called "thick accumulation and thin hair" and the so-called "one minute on stage, ten years off stage" are all said by the history teacher in the story. "Life preparation". What have you prepared? It is the so-called "bucket of water" or even a pool of water. This water is the professional knowledge consolidated by in-depth study and all kinds of knowledge accumulated by extensive reading, that is, the "expert" and "learned" "How can the canal be so clear? Only flowing water comes from the source. " To have a good class, there must be a steady stream of water.