Think about a time when you were a part of a group. What type of a group was it? What did the group do together? What was your role in the group?

I was in a class, we learned together and my role was to be a student.

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.

not concrete, existing in thought or as an idea but not having a physical or concrete existence