一、主题关键词，总是会出现和发展、起源或者历史相关的词汇(e.g develpoment, change, origin, historical)
【Professor: And what instrument comes to mind when you think of rock ‘n' roll?
Student: The electric Guitar?
Professor: Exactly. I think it's fair to say that the sound of the electric guitar typifies the rock ‘n’ roll genre,which became popular in the 1950s. But really the instrument we know today was the result of a continuing development that started for our practical purposes in the 1920s. But long before that even,people were experimenting with ways to modify traditional acoustic guitars.】
注意到文章开头做了两件事情：一. 提出话题“ Electrical Guitar”; 二. 提出了历史类文章要素, 教授说到电吉他是一个“the result of a continuing development” 而且人们很早之前就开始“ modify acoustic guitar”，自然而然我们就能推测文章是要讲电吉他是如何发展起来的。
【The first guitars were wooden. This is the Spanish guitar and the strings were made from animal products. Then came steel strings. And that led to the lap guitar,which is also called the steel guitar because the player slides a steel rod up and down the neck. And those are all acoustic guitars. OK?
But then eventually we have electric guitars. Over the years,many inventors and musicians contributed to the design of these instruments. And each design was intended to alter the sound in some way,at first at least with the electric guitar,to make it louder.