Why does it rain?

The sun heats the water in oceans, lakes, and puddles, and tiny bits of it lift up into the air as invisible water vapor. Higher up, the air is colder, so the vapor cools down and clumps into tiny water droplets, which is what clouds are made of. When enough droplets stick together to get heavy, gravity pulls them back down as rain. The same water keeps going around and around: ocean to sky to land and back.

Introduced water vapor, droplets, and the water-cycle loop. Skipped condensation nuclei (raindrops need a tiny dust or pollen speck to form around) and the role of updrafts and temperature lapse rate in storm formation. Save for a deeper conversation, or for the kid who asks about thunderstorms.

Solar energy drives the hydrological cycle by evaporating surface water into atmospheric vapor; the vapor rises until it cools below its dew point and condenses around aerosols (dust, salt, pollen, even bacteria) into cloud droplets ~10 microns across. Those droplets stay suspended until they grow, either through collision-coalescence in warm clouds or through the Bergeron process (ice crystals growing at the expense of supercooled droplets) in cold clouds, until gravity overcomes air resistance and they fall. The convective updrafts that produce thunderstorms can suspend droplets for unusually long, which is why thunderstorms produce heavier rain and occasional hail. A common misconception is that rain falls 'when clouds get too full'. Actually most clouds never rain at all; they need the droplet-growth mechanism to kick in.