According to China's traditional theory of five elements, the five elements include wood, fire, earth, gold and water, which restrict and restrain each other. The word "water" in the word "sea" represents the concepts related to water travel, such as water flow and water wave. Water represents coolness, fluidity, softness and other characteristics related to water element. According to the theory of five elements, the word hai belongs to the waterline. This attribution is based on the observation and summary of the relationship between the five elements, as well as the interpretation of glyphs and symbolic meanings.