Water—refreshing, soothing, cleansing water—covers more than 70% of our planet’s surface and is the only substance in nature to appear naturally in all three common states of matter: solid, liquid, and gas. We know water as the nuclei of comets in space; as vapor in the soft clouds of our blue sky; as the crystals that form the many intricate patterns of snowflakes; as vast glaciers atop our planet’s most tempting and challenging mountains; as brilliant, white icebergs drifting freely in artic oceans; as freshwater rivers that course through our land; as aquifers in the soil beneath our feet. On the molecular level, water can form large numbers of hydrogen bonds, giving rise to an unusually high surface tension and extreme capillary force, which, in turn, allows water to move upward through narrow tubes against the force of gravity—a life-giving property relied upon by all vascular plants, including our greatest trees: aspens, oaks, and redwoods. Fascinatingly, the boiling point of water is highly dependent upon the barometric pressure of its environment.