Short answers, pulled from the story.
Water is an inorganic compound of hydrogen and oxygen, and it is often called the universal solvent because it dissolves more substances than any other liquid. Its strong polarity relative to its small molecular size lets it dissolve many salts, sugars, and simple alcohols, though it is poor at dissolving nonpolar substances.
Ice floats because water becomes about 9% less dense when it freezes, reaching a density of 917 kilograms per cubic meter compared to liquid water's maximum density of 999.972 kilograms per cubic meter at 3.98 degrees Celsius. This floating ice insulates the water below and prevents lakes from freezing solid, which protects most aquatic organisms through winter.
Water covers about 71% of the Earth's surface, and seas and oceans make up roughly 96.5% of the total water volume. Groundwater accounts for about 1.7%, and the glaciers and ice caps of Antarctica and Greenland hold another 1.7%.
The U.S. National Academies of Sciences, Engineering, and Medicine recommend a daily intake of 3.7 liters for adult men and 2.7 liters for women, with about 20% of that coming from food. The popular claim that a person should drink eight glasses of water per day appears to have no real basis in science.
All known forms of life depend on water, which acts both as a solvent for the body's solutes and as an essential part of metabolism. It is central to photosynthesis and cellular respiration, and the human body is on average 50 to 60% water.
Water has been detected throughout the Solar System and the wider universe, including a cloud of water vapor around a quasar 12 billion light years away that holds 140 trillion times more water than all of Earth's oceans. Within the Solar System, liquid water exists beneath the surfaces of Saturn's moons Enceladus and Titan, and NASA detected water molecules on the Moon in September 2009.