Great Science Fair Projects
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In order for your area of interest to lead to good project, you need go from a general topic to a specific question you can answer. If the specific question is not perfectly clear right now, that's alright – you just need to do a little more research (technically, research is the next step, but it's o.k. to mix these two together).
You will need to be sure that your question is not too general. It needs to be specific enough that you can research it and try to answer it with an experiment in the time you have.
Also, make sure that your question is hard enough that it is worth answering (that's not all that hard at all, really!), but not so hard that you cannot answer it. It is a good idea to get some feedback from your teacher or mentor at this point.
Your question should be open ended – not a simple yes or no question. A good science fair project (like good science) should lead to extended discussion, conclusions and more questions.
Also, make certain that you can answer your question without breaking any of the science fair's rules, like using dangerous materials or harming animals.