A good testable question should be able to be answered by designing and conducting an experiment. Moreover, while choosing a question, a person should consider topics that are interesting.
A science fair project offers an easy way to answer a testable question, which should be the backbone of a person's work. The question should be interesting to allow a person to research on within the provided period and it's easy to solve with a simple experiment. If a teacher offers a specific area for the project, a student should first source for several ideas on that particular area. This will allow one to narrow down on one specific question that is interesting.