Question

How does rain fall from the sky?

Answer 1

Well gravity eventually affects the clouds as they swell and turn from condensed water vapor to droplets, and comes down as rain.

Answer 2

Rain - and other forms of precipitation - occur when warm moist air cools and condensation occurs. Since warm air can hold more water than cool air, when the warmer air is cooled the moisture condenses to liquid - and it rains.