Explain the process and purpose of controlling variables in scientific experimentation. Help students think through the variables that could affect the growth of a plant. These could include quantity of light, air and water; ambient temperature; and quality and type of liquid and soil. Do your best to let the students think of as many of these as possible. Help each student pick one variable to vary and predict the results. Give each student four cups, four seeds, and access to soil, water and light.
Students who wish to explore how watering affects plant growth may vary either the quantity or type of liquid used. Help them plant their seeds at uniform depth in the same type of soil and find a position that will give each plant the same amount of sunlight. Those who want to vary the quantity of water should pick one quantity for each cup and add that much each day. Others may want to add sugar water at various concentrations, different fruit juices or varied concentrations of plant food in water.
Students who wish to vary the quantity of light available to each plant should design small cardboard boxes that block out various amounts of light. One box could simply be covered, with air holes poked in the back near the bottom to keep access to air relatively constant. The others could have circles of steadily increasing diameters cut out of their covers. Another option is to cover one box with a single sheet of paper, the next with two, and so on.
Give students leeway to test the effects of other variables if they would like to. Even if a certain variable will have no measurable effect, the experiment will be successful if the student discovers that fact. Letting students pursue the questions that interest them will engage them in the process of scientific inquiry. Interested students can test the effects of wind speed by training fans at different settings on their plants or soil acidity by mixing distilled vinegar into some pots. Other students may want to explore the effects of music, temperature or even pot color.