The water we use today was around in Roman times. Liquid water evaporates into the atmosphere from oceans, rivers, and puddles. It evaporates from wet laundry drying in the sun. 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.