Density is the amount of “stuff” (molecules and atoms) that are packed into one space. Objects’ density can be found by dividing the mass (weight) of an object by the volume (space it takes up) of an object. Density is measured in grams per milliliter (g/mL). If something has to float on water, than the density would be smaller than the density of water. If something has to sink in water, it’s density is greater than the density of water. Different materials have different densities. For example, gold’s density is 19.3 g/mL. Water’s density is 1 g/mL. When things are heated up, their density gets less and if things are cooled down, their density gets higher. That is how density changes!