點解承托既空氣愈多, 密度就愈細愈浮得起?

Substances will float if their density is less than the density of water, which is about 1 g/mL.
By definition, density = mass /volume
For a given mass of substance, if its volume is made larger by introducing an empty cavity in it, its density will decrease.
In such a way, when the volume of the substance is so large such that its density is reduced to less than that of water. It will float.