Good science fair projects could fall under a number of categories, including environmental science, astronomy, chemistry, and cooking/food science. For example, you could look at how light intensity from stars changes with distance.
Your school website is a good website to find a science fair project. Science programs for kids typically have websites with projects listed. PBS has a website for kids that includes science projects.
The back of your science books often has ideas you can use,
go to the site Google.com and ask good science fair projects
I always loved coming up with science fair projects with my children while they were in elementary school. A good one for a third grader are playing two different kinds of music to two different plants to see which one grows faster and making a daily log regarding the plant and the music.
Try artskills.com and you can get some good ideas there for school projects and exc. good luck !!
sciencebuddies.com was recommended to me by my science teacher and it is a very good website that can help you with science projects and even help you decide what you want your project to be!!
There is actually a very good site focused on science projects called sciencebuddies. This site is focused on useful projects. However, other good sites include World Book and sites focused on specific topics such as chemistry.
google.com or science fair projects.com
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Math And Science in Harrisburg is a very good school. (Based on the corrections we had to make to this answer, we wonder...)