Allow students to make predictions based on their previous observations. Pill bugs prefer apples to lettuce. Ask students to develop ways to test their questions. Creating a data collection table may help students organize the information as they gather it. Keeping variables constant is a challenge for scientists.
Students usually understand the concept of being fair.
The analogy of a group of people playing the same game by different rules often illustrates the importance to students. Once you have a constant environment, THEN you can begin changing individual factors. This will give you reliable information. Your results will be based on the one variable you are studying, not other outside influences. Allow time for students to plan and conduct their research project.