John's teacher gave all the students the following materials: three seeds, potting soil, water, a light source, fertilizer, and a ruler. She told the class to make a question and a prediction using those materials. The following statement is a prediction John made over the watering plants project his teacher gave his class. "I think giving plants 30 milliliters of water everyday will cause the plants to grow taller than giving plants only 10 milliliters of water a day." What would you infer John would be measuring after evaluating his prediction?