To make it more specific to what Qi En said, get two potted plants with the same type and amount of soil.The plants must also be the same height.First,put both plant to a place where sunlight is shining at.However,you must put each plat 100m away from each other.After that,use a measuring cylinder and put 100ml of water into.Then,pour it into both plant but put 50ml of detergent to one of the plant.Do all this steps for 7 days and record it everyday and after 7 days,see which plant grows faster.