A group of students wants to investigate the effects of plant moisture on the growth of caterpillars. The students fill three identical boxes with an equal number of a kind of leaf that caterpillars eat. The students keep the leaves in the first box dry, spray the leaves in the second box with two sprays of water, and spray the leaves in the third box with four sprays of water. The students then add four caterpillars to each box. Every day, the length of the caterpillar is measured, recorded and the results are averaged. What is the tested (independent) variable in this experiment?