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 important 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.