Everybody knows that water flows downhill. The reason is that the force of gravity pulls everything downward toward the center of the Earth. If a material is resting on a sloping surface, part of the force of gravity acts in the downslope direction. If that does not make sense, think about leaning a board against a wall at an angle and placing a brick on the board. You need to exert a force to keep the brick from sliding down the board. The steeper the board, the more force you have to exert to keep the brick from sliding.

In rural areas, most homes and businesses get their water from groundwater. Long ago, wells had to be dug by hand and reached only shallow groundwater. Now wells can be drilled by machinery to as deep as several hundred feet. In urban and suburban areas, most water is piped in from a central water supply. The water supply might be a river, a natural lake, a reservoir behind a dam, or a number of deep wells. In many big cities, reservoirs are located far away, and the water is brought to the city through aqueducts.

In most parts of the world, water is a scarce resource. That might seem strange, because there is so much water on Earth. Almost all of the water on Earth, more than 97 percent of it, is seawater in the oceans. The rest is called fresh water, because it does not have a high salt content. Most of the world’s fresh water is frozen solid in large glaciers in Antarctica and Greenland. Almost all of the fresh water that is available for human use is either contained in soil and rock below the surface, called groundwater, or in rivers and lakes.