The Wu Xing ( ), or Five Elements, are used in many traditional Chinese fields such as traditional Chinese medicine and martial arts. In medicine the Five Elements describe the interaction between internal organs and medicinal properties. The Five Elements system has been in use in its current form since the Han dynasty, in the second century BCE. The usual order is wood, fire, earth, metal, and water, known as mutual generation or generating cycle: wood feeds fire, which (as ash) creates earth, which produces metal, which channels water (e.g. by pipes), which nourishes wood to complete the cycle. The sequence of mutual overcoming or destruction is wood, earth, water, fire, metal (wood in the form of tree uses earth, earth absorbs water, water destroys fire, fire tames metal, metal e.g. as axe destroys wood). This overcoming sequence means the visual representation is often in the form of a pentagram.