In Chinese, "wu" refers to five categories of things in the natural world, namely wood, fire, earth, metal and water; "xing" means movement and transformation. So "wuxing" (the five elements) actually refers to the movement and transformation of these five elements as well as their interrelationships. The theory of the five elements holds that all things in the natural world are derived from wood, fire, earth, metal and water. So they all bear the basic properties of these five elements and maintain a harmonious balance through the activities of constant inter-promotion and inter-restraint among the five elements.