单选题 Suppose you have a brick and a sponge which are exactly the same size. The brick and the sponge will occupy the same amount of space, but they will not weigh the same. If two things are exactly the same size, but differ in weight, then they have different densities. The one with the greater weight for the same size has the greater density. The one with lower weight for the same size has the lower density.
Now apply this idea to fluids. You know that water and air expand when they are heated and therefore become less dense. If you poured a cupful of very hot water and let it cool, you would have a little less than a cupfull of cool water. If you weighed a cupful of very hot water and a cupful of cool water, the cupful of very hot water would weigh a little less.
The cupful of cool water, being a little heavier than the cupful of hot water, would really have a little more water in it. Cold water is denser than hot water. Likewise, cold air is denser than hot. air. The greater weigh! of !he denser material makes it fall to the bottom of the container. It pushes up the less dense material.