A useful science fair project can be made out of an experiment that you decide to conduct. However the crux of the matter says that the experiment should do something useful such as answer a specific question. It does not have to be a complicated question that no one has ever answered before.

You can happily leave such complicated stuff and experiments to professionals, but is it not possible for you to come up with a question about something around you? A question that can be answered by an experiment that carefully monitors data? You can do something as simple as watching and recording to saplings grow in different parts of the house.

The basic question here would be something as simple as do plants kept in a sunny room grow faster and healthier than plants kept in a room without a window. Now get the two identical plant saplings and get started with keeping a record of their growth each week. Keep all the data ready in your journal to make attractive charts and graphs.

Study all the differences that appear on the physical aspect of the plants. Record any specific incidents that caused a spurt of growth for a particular plant. And there you have it a great comparative study in plant growth answering a specific question. It may not be an exciting science project but it will be a useful one to prove a fact.