@Dutchess_III That’s a very interesting question. There’s lots of water on Earth. 75% of the planet’s surface is covered with it. The surface water gets evaporated into our atmosphere by solar radiation. As it cools in the upper atmosphere, it generally condenses out of the atmosphere to freeze on microscopic dust or even bacteria in the air. More and more ice then builds up, eventually getting so heavy it falls and, it it’s warm enough in the lower atmosphere, melts back into rain.
That leads to the question, where did all this Earthly water come from? Some would have been here from the planet’s beginning. Sometime back in the distant past, well over 4.5 billion years ago, a large star or stars exploded in a supernova leaving a massive cloud of dust and gasses. Gravitation pulled this debris field back together over time, and our sun came to life. The gravitational pull of the sun caused the rest of the dust and gasses to begin orbiting in an ever more disk-like form, and eventually bits gathered into ever larger chunks within that disk, forming planets and moons. Where there was not enough material to form a planet, we ended up with asteroid belts.
In our planet’s early life, and again later, something disturbed the equilibrium of the asteroid belts, causing a large number of comets and asteroids to crash into the Earth in what is called the Early and the Late Bombardment. Asteroids and comets also often contain water, so the two bombardments left us the beautiful, sapphire blue gem of a planet we now enjoy—at least when it doesn’t rain too much. :-)
Now others have mentioned that water originally comes from the combining of two highly reactive elements, hydrogen and oxygen. Those two have an enormous affinity for one another. Whenever they can, they get together, sometimes explosively so. The Big Bang, as its extreme heat cooled, created only hydrogen and helium from all the quarks, leptons and gluons that it gave birth to. For oxygen to come along, we had to wait till gravity brought enough of the first two gasses together to give birth to the first large stars, having at least 10 times the mass of our sun and sometimes thousands of solar masses. Such large stars begin to undergo gravitational collapse when they consume all their hydrogen through nuclear fusion. As they fall inward into their core, core temperatures rise astronomically till first helium fusion, then heavier element fusion begins. In their final death throes when all this fusion slows till its pressure can no longer resist the forces of gravitational collapse, it triggers a violent explosion known as a supernova. That blasts all the heavier elements occurring in nature, including precious oxygen, into space in the form of a gas and dust cloud know as a nebula.
And it is from such a cloud of ancient star dust that we, and the oxygen in our water, and the planet sustaining us and Sun warming us all came. So rain comes from stardust, and so do all of us.