Select two or more treatments of the same issue, problem, or research. For example, you might bring in an article on a new diet drug from USA Today, The Wall Street Journal, and the Journal of Dietetics. Ask students to write about what constitutes proof or facts in each article and explain why the articles draw on different kinds of evidence, as well as the amount of evidence that supports stated conclusions.
Alternatively, ask students to look at a range of publications within a discipline--trade journals, press releases, scientific reports, first-person narratives, and so on. Have them ask the same kinds of questions about evidence and the range of choices writers make as they develop and support arguments in your field.