"Cubing" or The Action Verbs Technique
One way to move from a "vague idea" to a doable paper topic is to write your vague idea at the top of a page, and then, briefly:
Do any of these aspects seem especially useful with regard to your topic? Do any common themes appear? Keep going back to it as you read up on your topic:
Contains in-depth, analytical reporting on the most current and controversial issues. Provides extensive background information and discussion of the pro’s and con’s.
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A database of encyclopedias, almanacs, and specialized reference sources for multidisciplinary research.
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Sources listed under Background Sources also make good places to get topic ideas.
Your paper's focus and thesis should strike you and your reader as interesting.
Determining what is "interesting" depends on both topic and scholarly field, but it usually involves something the reader has some prior knowledge about, but with a twist: a previously unexplored aspect or an unexpected finding. Another way to put it is that interesting is the middle ground between obvious ("duh") and meaningless ("huh??").
One way to think about an interesting approach to your topic is to think of your topic in terms of a contrast:
Any of the above patterns can work in reverse as well.
For example: Something that appears to be insignificant may turn out to be significant: "Contrary to popular belief, there is actually a significant relationship between stretching before exercise and reduction of injuries during exercise."
More examples of thesis statements and how to write them: