I agree with AvidLearner. I also believe that the definition of a successful science fair project may be determined by the motivation level of the student. That is, some students hope to take their project all the way to the regional or national science fairs, while others are content to have some fun and complete their project in time for the school science fair. Whatever your level of motivation, there are many sites available to help define an idea and thoughtfully complete the steps necessary to put together a high-quality experiment.
Education.com has a science fair section - http://www.education.com/topic/help-child-with-science-fair-projects/. We also have a section of the site dedicated to science activities - http://www.education.com/activity/science/ - many of which can be used for a science fair.
Good luck!
L. Compian, Ph.D.
Counseling Psychologist
Education.com Reference Team
Take a look at these K-12 top 12 science project websites, where I describe the strengths and features of free websites that offer resources for science fair project and experiments:
http://www.squidoo.com/k12interactivescienceprojects
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