But before water can fall back to earth as rain, you need clouds. Dust particles serve as tiny space stations that attract water molecules. Raindrops are just grown-up cloud droplets. Cloud droplets grow through condensation, and by bumping into one another and merging. Condensation is what happens on the outside of a drinking glass when it's filled with icy water on a warm day.
Humid air hits the cold glass, and water droplets form on dust particles.
Warm, moist air rises through the atmosphere, just as hot-air balloons rise. Cloud droplets form. After a while, those drops get too heavy to stay in the cloud. Then they fall. You may also be surprised to learn what raindrops look like.