	Indigo, a plant used to dye clothing, which could make the cloth blue. It has been regarded as an ingredient of dye for a long time. In "Encourage Learning", an article of Xunzi, indigo was mentioned as a metaphor that students will be better than teachers.(Chinese: 青出于蓝而胜于蓝) In 1940-1956, the output of indigo continued to increase. However, after that time, with the importation of foreign dye, the production of indigo has been decreased. Nowadays, there are only a few people planting indigo.