Consider how a kettle boils off water by heating it up, transforming it into a gaseous state which diffuses it's mass, changing how it interacts with gravity.

As the water cools, it will tranform from gas to liquid, again undergoing changes in mass and subsequent behavioral change under gravity. The result in condensation.

Heavy clouds are full of gas which get released when that gas state changes into liquid. The process of clouds raining is the same as condensation.