We can answer this question from two aspects: for the first paragraph, structurally speaking, it is to write down the topic, point out the center of the article, get straight to the point, get the full text, or play a role in causing the following; In terms of content, it is the foreshadowing and foil for the following, paving the way for the description of the following content. The problem in the middle part plays a bridge role in the structure. The function of the last paragraph or sentence is to summarize the full text, point out the main idea of the article, make people remember deeply and take care of the theme.