A group of students does an experiment to see if different colors of light affect the number of flowers a plant produces. Their teacher gives them seeds, pots, soil, water, and three light bulbs of different colors. The students decide to divide the materials equally. What should be different for each group of plants?

The questions have been asked in several important state and regional science exams in the United States. For example: TAKS-Texas Assessment of Knowledge and Skills, New York State Educational Department Regents exams, California Standards Test, etc.