 We have a hollow aluminum cylinder floating in some water. We will heat the water. As we heat the water, we increase the kinetic energy of the molecules in the water, and they move further apart from each other, and the density of the water decreases. The aluminum metal, however, does not change volume very much with temperature and does not change its density significantly. As the temperature increases, eventually the density of the water will be less than the density of the aluminum, and the aluminum will sink.