A student hypothesizes that pillbugs are attracted to darkness. To test the hypothesis, 20 pillbugs are placed in the middle of the a pan in which one half of the pan is covered with paper to make it dark. The pillbugs are then allowed to wander around for 10 minutes. What is the experimental variable in this experiment?

In an experiment, the variable is the component, which is changed, in order to observe the behavior of the setup.

In this case, the only component, that is changing is light. In the whole experimental setup, other things such as pillbugs, pan etc are constant, but half of the pan is having a normal amount of light and half of the pan is dark.

Hence, the experimental variable in this case is light.