A student is testing whether light is required for corn seeds to germinate. He plants one group of seeds in soil in the dark and leaves the other group exposed to light, sitting on moist paper towels.

The intended independent variable is light exposure: one group is kept in the dark and the other in light. But the groups also differ in what the seeds are planted in—soil versus moist paper towels—so the growing medium is an uncontrolled second variable. Because of these two differences, the student cannot tell whether any difference in germination was caused by light or by the medium.

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