Now it's time to think about how best to set up your science fair experiment to give you accurate and trustworthy results. You should keep in mind both the Scientific Method and the question you are trying to answer.
An experiment is made up of two nearly identical parts--let's say, two trays of tomato plants. The tiny differences that will test your hypothesis are called "variables." Let's look at the three kinds of variables.
The Independent Variable: This is a variable that you intentionally change. In the case of our tomato plant experiment, it would be the brightness of the light in Tray #1.
The Dependent Variable: This is the difference between the two parts of the experiment that happens when the independent variable is changed. In this case, it would be the size of the tomatoes in Tray #1. So you turn the up the lights in Tray #1 (the independent variable) and the tomatoes get bigger (dependent variable).
The Controlled Variables: These are the variables that are kept the same in Tray #2. In other words, when you turn the lights up in Tray #1, the lights in Tray #2 stay the same. So when the tomatoes in Tray #1 get bigger, you can say that the difference in size between the tomatoes in Tray #1 and Tray #2 is the result of the independent variable, or the light that you made brighter.
Tray #2 is called a "control group." This is an identical setup to Tray #1, but it is kept in its natural, unchanged state. Tray #1 is called the "experimental group." So when you change something in the experimental group (like the brightness of the light), the control group stays the same so that you can measure changes in you dependent variable (the size of the tomatoes).
When you design your science fair experiment, you have to keep as many things identical as possible. For instance, the lights that you use to grow the tomato plants have to use the same light bulbs. The trays have to be the same, just like the soil has to be the same. The amount of water has to be the same too.
You should think very hard about differences that might try to sneak by you. For instance, when planting the tomato seeds, did they come from the same package? They should if you want to keep things under control. The only thing that can be different between the two identical groups is the thing that you are testing - in this case, light. The exact nature of your variables will depend on your exact science fair experiment idea.
If you changed the amount of light, AND changed the amount of water each plant received, you wouldn't know if the bigger tomatoes were because of the light, or the water! So it's absolutely critical to have everything be as close to identical or "controlled" as you can get it.
Once you've done your science fair experiment, consider repeating it if you have the time! This is very impressive for science fair judges, and really helps to make your results convincing. If you conduct your experiment and the results just don't seem right, you might consider reviewing your experiment idea, and possibly even modifying your experimental design so that your science fair experiment will produce accurate results.