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.