Why don't clouds fall down?

Because as my earth science teacher always told me "Warm wet air rises"

It's a density related matter, and as clouds are less dense than air, they float in the sky rather than falling down.

More: Think about this: Liquid helium doesn't fall up and splash on the ceiling.

Clouds are LIGHTER than air, and the barometer indicates LESS air pressure on a cloudy day because of that.

Water vapor is H2O which is Half the mass of air molecules (N2 and O2) because it has only one O (and two hydrogens which are even lighter than one helium atom).

Clouds are not liquid, and you can't swim or drown in them, unless they become liquid and then a lot of Rain falls down.

Clouds are not solid, and you can't walk on them, but if it snows, the snow may be something like 10 times deeper than if it had rained.

Clouds are volumes of relatively concentrated water vapor.

A volume of water vapor weighs only half of what a volume of air does.

Water is common but strange stuff, because as a liquid it is heaver and denser than it should be, and when its a gas or a solid, it is a lot lighter and less dense than it should be, and that's not obvious.
Well because they have already fallen down and are lying on the ground. Due to the vertically changing air pressure, temperature and water density the lower cloud part is mainly transparent( and therefore invisible ) and the top part like a fog respectively. The higher cloud part is transparent again.

If some piece of foggy cloud descended and crossed the bottom boundary of the visible cloud, the environmental conditions ( mainly the pressure ) let this cloud piece to transform to invisible vapour and vise versa.

Sometimes for the same reason there is a pack of bigger number the transparent and nontransparent layers -- so called multilayer clouds. Due to the approximately exponentially falling air pressure with the height the bottom boundary of visible clouds seems often to be more flat ( plain ) in comparison with the top cloud boundary.