A raindrop or snowflake that hits the upper slopes of the Big Horn Mountains joins other drops, gains momentum, rolls over Shell Falls, flows west into the Bighorn River, then north into the Yellowstone River in Montana. Near the North Dakota border it meets the Missouri River, then travels south and east through the farmlands and cities of the Midwest before the Mississippi River carries it through the Deep South into the Gulf of Mexico, a 4,000-mile journey. Yet the trip is rarely so simple or direct and never really ends, because the drop moves over, under, and around the earth in the never-ending water cycle, and water flowing over Shell Falls today could wind up in a bathtub next month.