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.