Water is the most common substance on Earth, covering almost three quarters of the planet's surface. Known by its chemical symbol, H₂O, water is the only known substance on Earth that naturally exists as a gas, liquid, and solid. The vast majority of water, about 97%, is in the oceans. The liquid form of water also exists in lakes, rivers, streams, and groundwater (water beneath Earth's surface that is held between soil particles and rock, often supplying wells and springs). In its solid form, water makes up sheets of ice on the North and South Poles, and permanent snow. Water also exists as water vapor (gas) in the atmosphere. The hydrosphere is the whole body of water that exists on or around Earth, which includes all the bodies of water, ice, and water vapor in the atmosphere. All life needs water to survive and...