Why does it rain?
To understand why it rains we need to recall both that there is water in the Earth, and the sun a few light minutes away, enlightening and warming up the planet.
With these two concepts in mind, understanding why it rains will not be hard:
+ First, the sun will come out and evaporate some water with its heat.
+ These tiny drops come together, creating clouds which wander around the sky.
+ Days after, when weather conditions aren't as warm, these small drops of water cluster together and form bigger droplets of water called raindrops.
+ At the time these drops get too heavy to be held by the clouds' own gravitational attraction, or the weather is not hot enough to re-evaporate them, they fall back to the earth one after the other, in what we call rain.
Up to there, the basic concept. It wasn't that hard, was it? I think there is only one thing to add to finish: Sunlight is not the only cause of hot or cold weather, or the evaporation or precipitation of water as rain. Winds, air pressure, and even pollution are involved too.