This doesn't sound like a science fair project but rather a homework question (which we don't provide help for). I suggest that you try submitting this question to a search engine.
If it is a science fair question, why don't you look at the guide on the scientific method/science fair at:
http://www.sciencebuddies.com/mentoring ... ndex.shtml
and then post back with some project details /hypothesis and we will try to help.