I think its because of the feed back you receive because from there you have a chance to fix any part of your paper.<span />
The main idea of this paragraph is 'A PRISON TAINT WAS ON EVERYTHING THERE'.
The main idea of a paragraph is also called the topic idea of that paragraph. The main idea of a paragraph is usually the most important sentence in the paragraph and all other sentences in the paragraph serve to support it.
Topic idea of a paragraph are usually the opening sentence in the paragraph.
In the passage given above, you can see that the main idea of the paragraph is the prison taint that affects everything that is found in the prison environment, all other sentences in the paragraph build on this main idea.<span />
