* Density is mass divided by volume (Density = Mass / Volume).
* Heating the balloon doesn't change its mass. The air inside just gets hotter and expands.
* Since the volume doubles, the density is halved. This is because the same mass is now spread out over a larger volume.
In simpler terms: Imagine you have a bag of marbles. If you spread those marbles out over a larger area, the density of marbles per unit area decreases.
