Density is the measure of how much mass of a substance is squeezed into a given volume of that substance. <em>It is the mass per unit volume</em>, and substances with lesser density will float in materials with denser density. Buildings are generally more obviously denser that air, if not we'll see then float upwards into the atmosphere, but that is not the case. Different liquids too can separate and form layers on one another due to their differences in volume.
