An experiment I have used before with success in my class room is to demonstrate the effect sunlight has on the growth of plants. You will need 6 plants, already in potting soil, and a place to let them collect sunlight. You divide the 6 plants into 2 groups of 3; one of these groups will be the experimental group, while the other is the control group. The control group gets equal amounts of water and sunlight. The experimental group, however, one plant gets full sunlight, while a second plant is given a limited amount of sunlight (1 hour/day), while the 3rd plant is placed in a totally dark area, with no sunlight at all. The time duration for this experiment is about 1 week.
What you will observe is the plants that received full sunlight are vibrant and healthy. The 2nd experimental plan has a sickly yellowis-green color, and the 3rd experimental plant is almost dead, with a thin, spindly appearance. This experiment demonstrates the connection between the amount of sunlight a plant receives and how well the plant grows. Care should be taken to make sure all the plants receive the same amount of water each day.