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)，举例来说：
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.
很明显的，教授接下来就开始讲到吉他的发展过程，第一种吉他，wooden aka spanish guitar;接着steel guitar;lap guitar;然后有了electric guitar。而且重点说到“ 很多的发明者和音乐家对这种乐器的设计做出了贡献，而且每次的修改的目的就是为了改变电吉他的声音，也就是要素中提到的对话题的影响。
So let’s get back to when the steel guitar was first introduced in the United States. It was right after the Spanish-American war in the late 1890s. US sailors who were stationed in Hawaii—then a US territory—were very enamored with the music they heard there. Uh,Hawaiian music was based on the steel guitar I just described. Some sailors learned how to play the steel guitar and brought it home to the States. Before long,Hawaiian steel guitar music was all the rage in the mainland US. It actually had a strong influence on the development of several musical genres,rock ‘n’ roll most notably,but also jazz and blues.
Anyway,by the 1920s,with the advent of the public dance movement,people were gathering in large groups to listen to steel guitar music. But they had trouble hearing it,especially in large public settings. As I mentioned,the instrument was played horizontally,on the lap. Since the strings faced upward,the sound was projected toward the ceiling rather than outward toward the audience. Something had to be done,because the music venues and the audience kept getting larger and larger.
Electrified guitars already existed by the time Les Paul came into the picture around 1940. What Paul did was experiment with ways of removing the distortions and he succeeded. He designed a guitar with a solid body that relied solely on electronics. Paul’s solid body eliminated the vibrations,and thus the distortions.
此处教授讲到Les Paul此人对于电吉他的修改，使得电吉他有了一个”solid body” ， 减少了声音失真的状况。