Why does it rain?

Rain is one stage of the water cycle, the endless circulation of water between the Earth's surface and its atmosphere. It starts with evaporation: heat from the sun turns liquid water from oceans, lakes, rivers, and soil into invisible water vapor that rises into the air (plants add moisture too, through transpiration). As that warm, moist air rises, it cools, because air gets colder at higher altitudes, and cool air cannot hold as much water vapor as warm air. So the vapor condenses, turning back into tiny liquid water droplets (or ice crystals) around microscopic particles of dust, pollen, or salt in the air. Billions of these tiny droplets clustered together are what form clouds. This is condensation, the reverse of evaporation.

Rain falls when those droplets grow big and heavy enough. Inside a cloud, droplets collide and merge, growing larger and larger, and when they become too heavy for the rising air to hold up, gravity pulls them down and they fall as precipitation, rain if it stays liquid, or snow, sleet, or hail if it is cold enough to freeze on the way. The fallen water collects in oceans, rivers, and the ground, and eventually evaporates again, repeating the cycle endlessly. This recycling is why the same water has been circulating on Earth for billions of years: the water in today's rain has fallen and evaporated countless times before, which is the basis for the (true) idea that you are drinking water that once flowed through dinosaurs and ancient oceans. So rain happens because the sun evaporates water, the water rises and cools into clouds, the cloud droplets merge until they are heavy, and then they fall, a continuous solar-powered engine moving water around the planet.