The purpose of a topic sentence is to introduce the topic and the main idea of a paragraph. In some essays, a topic sentence often contains a "claim" about its topic, which serves as the main idea.
To be effective, a topic sentence should be focused, not too broad. The following topic sentences are too broad:
1. The United States has many museums with excellent collections of art. (You could write a book about all the museums in the United States, not just one paragraph!)
2. Cultures vary throughout the world with respect to body language. (This HUGE topic could also fill a thousand pages of a book. It is too much to cover in one single paragraph.)
Now, look how these two topic sentences are focused and appropriate for paragraphs:
1. The St. Louis Art Museum has an outstanding collection of Expressionist paintings and prints. (This topic sentence indicates that the paragraph will discuss only the St. Louis Art Museum's Expressionist paintings. This is a manageable subject for one paragraph.)
2. Hand gestures that are seemingly innocent in the United States are obscene or insulting in certain Latin countries. (Much better. This paragraph will focus on only hand gestures, not all forms of body language. Also, the paragraph will only compare hand gestures in the United States to those in Latin countries.)
A topic sentence should also be capable of being developed, and, therefore, should not be too factual or narrow in scope:
The following topic sentences are merely factual and do not allow for development:
1. St. Petersburg is a major city in Russia.
2. California has more than one hundred community colleges
Now, note how these topic sentences are revised so that they can be developed within a paragraph:
1. St. Petersburg reminds its visitors of the Italian city of Venice.
2. Many California college freshmen prefer the community college for its many unique features.