Sam believes the rock is more dense than the pencil. He measured the mass of the rock to be 8.5 grams and volume to be 4.5mL. He measured the mass of the pencil to be 2.4 grams and volume to be 4.5mL. Density is the mass per unit volume of an object. When he put the rock and pencil into the water, the pencil floated and the rock sank. Which is more dense? Give your reasoning using actual density values.