Water doesn't stay in one place. It evaporates from oceans, gets carried inland by wind, falls as rain, flows downhill into rivers, and eventually makes it back to the sea. Then it does the whole thing again. This loop — the water cycle — has been running for billions of years, and it's been quietly deciding where humans can and can't survive the entire time. No water, no food. No food, no people. Every civilization that ever existed had to solve the water problem first. Everything else — trade, culture, war, art — came after.

The cycle works like this. The sun heats surface water — mostly oceans — and turns it into vapor. That vapor rises, cools, and condenses into clouds. Wind pushes those clouds over land. When conditions are right, the water falls as rain or snow. The water doesn't fall evenly, though. Mountains force air upward, cooling it fast and squeezing out rain on the windward side. The other side — called the rain shadow — gets almost nothing. This is why you can have a lush rainforest and a desert separated by a single mountain range. The Andes do this to South America. The Cascades do it to the American West.

Now it gets interesting for the modern world: we've partially broken the cycle. We pump groundwater out of ancient underground reservoirs — called aquifers — that took thousands of years to fill. Cities like Las Vegas and Phoenix exist in places the water cycle never intended humans to live in large numbers. They survive by borrowing water from the past. That works — until the aquifer runs dry or the river they depend on gets stretched too thin. The Colorado is already there. The old rule reasserts itself — the same one that put New York, London, Cairo, and Shanghai exactly where they are. Follow the water. Same as always.