Here are some websites to help you brainstorm a question for your Science Fair:
http://www.sciencebuddies.org/science-fair-projects/project_ideas.shtml#browsealprojects
http://www.sciencebob.com/sciencefair/ideas.php
http://school.discoveryeducation.com/sciencefaircentral/Getting-Started/idea-finder.html
http://www.factmonster.com/spot/sciproject2.html
http://www.energyquest.ca.gov/projects/index.html#chemical
http://www.sciencebuddies.org/science-fair-projects/project_ideas.shtml#browsealprojects
http://www.sciencebob.com/sciencefair/ideas.php
http://school.discoveryeducation.com/sciencefaircentral/Getting-Started/idea-finder.html
http://www.factmonster.com/spot/sciproject2.html
http://www.energyquest.ca.gov/projects/index.html#chemical