Three quarters of the earth's surface is covered by oceans, lakes, marshes, and rivers, with a total size amounting to around 1330million ㎦. Moreover, water exists in soil and rocks found under the surface of the earth, and in the form of underground water, amounting to almost 82 hundred thousand ㎦. This sea and earth water absorbs solar heat, creating roughly 130 million ㎦ of vapor that spreads throughout the atmosphere. The vapor is condensed and combines to form clouds and fog. These, in turn, transform and descend onto the earth's surface in the forms of rain, snow, or hail, which then merge with rivers that flow eventually into lakes and seas. This is the cycle of water. In the course of this cyclic process, water erodes the high and low altitudes on the earth's surfaces, alters the courses of rivers, and carries rocks or soil to faraway places. Furthermore, large amounts of rain or flooding corrode mountains, create deep valleys, and even cut shapes out of solid rock. Raging waves continuously hit the shores, changing the shape of islands and even continents.

People have a deep interest in water, an element essential for the survival of living organisms. Throughout history, water has been an important subject to philosophers. Around the 6th century B.C., Greek philosopher Thales advocated nonism, an ideology in which water was the basic element of the universe, and in which all other compounds were variants of water. This ideology seems to have originated from observations of the changes in the state of water to snow, fog, hail, ice, and vapor. In the 5th century B.C, Empedocles advocated the quadruple element theory, which was based on the idea that water, soil, air, and fire were the universal elements. Since then, the idea of considering water as an element has lasted up until about the 17th century.

For example, even J.B. Helmont, who first identified the presence of carbon dioxide in the air, had experimented to prove that plants only need water for growth. (At that time, the photosynthesis of plants had not yet been discovered). One person who disagreed with the idea of water being an essential element was German miner, G. Agricola, in the 16th century. It was then A.L. Lavoisier who proved that water was not an element. In 1768, Lavoisier accurately measured the weight of the soil left over after water had boiled and evaporated from a glass bowl, proving that the leftover soil came from the smelting of the glass bowl.