為什麼密度較小的, 會在密度較大的上面?

If you look at this question from the microscopic point of view, hot air with higher temperature have more kinetic energy, and hence higher speed, in its molecules. Cold air molecules have slower speed. Thus molecules with higher speed tends to [fly off] more easily and could remain staying above those with slower speed.

In the macroscopic point of view, density = mass/volume, a given mass of hot air has larger volume, hence lower density, and displaces larger volume of surrounding air than when it is cold.
The upthrust acting on this mass of air is thus greater than its own weight because of the larger volume of surrounding air it displaced(Principle of Floatation). Therfore, hot air rises above cold air.

地心吸力對密度大的物件的吸引力大，
所以密度大的物件會沉到密度小的物件下。