What is a topic sentence and what's a good example of one?
While I agree that a topic sentence introduces what a paragraph will discuss, it is not a "thesis" in any sense. Your thesis is your main argument. Each body paragraph then presents evidence and argumentation that supports the thesis statement. In a five-paragraph essay, there is the thesis paragraph, three body paragraphs--each of which presents one piece of evidence (for a total of three)--and a concluding paragraph. The topic sentence presents the evidence; that is why it's the topic of the paragraph. The remainder of the body paragraph interprets the evidence and explains how this helps to prove the thesis.