Agood writer is a good reader discuss

the purpose of a nonfiction text is to give the text a rap up. for example, when i say : frogs are interesting animals is an ending. then when i say: frogs are unique in different ways. that is an intro.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.