Katherine wants to investigate if the time of day a plant is watered affects the growth of the plant. She waters the same type of plants with the same amount of water at the same time in the morning. The plants are sitting in the same location and get the same amount of sunlight. Which variable is missing from the experiment?

Katherine wants to investigate if the time of day a plant is watered affects the growth of the plant right? If she waters the plants at the same time she is not doing the experiment. Although she has the same type of plants, the same amount of water, same location and same amount of sunlight for each of them, how is she going to find out what she needs to if she doesn’t water them at different times?