Marissa makes dye for fabrics by dissolving dye powder into water. She would like to find a way to more quickly dissolve the powder. She designs an experiment to see whether the particle size of the dye powder affects how quickly it dissolves. Marissa first gets a sample of blue dye powder with a large particle size. She grinds some of this powder to make samples with small and medium particle sizes. She fills three separate cups with 8 ounces of hot water. She then performs three trials. In each, she adds 20 grams of one of the dye samples to one of the cups. She stirs the mixture at a constant rate until she can no longer see the dye powder and the color is evenly distributed in the water. At this point, she records the time it has taken for the dye powder to dissolve. In each trial, why does Marissa add the same mass of dye powder to each cup?