Most communities could manufacture vital resources by themselves. However, sometimes they had to risk to find some other resources. Food: Surprisingly, plants evolved to endure the freezing cold temperature and they managed to grow under a temperature of -20 degree. However, if the temperature continued to stay low (for example, -40 degree), they would stop growing and hibernate until the climate was suitable. Water and coal: Coal was the fundamental resource of heat, as well as water. People obtained water through melting ice. Oil and fur: Oil was one of the few reasons for people to go out. They obtained oil from hunting animals, and the oil was then used in infrastructure or industry. Although people could manufacture cloth using straw and they could protect people from freezing cold, they were not durable and should be replaced within a few weeks. Therefore, animal furs, which were considered durable and convenient, were also irreplaceable. Salt: People could easily discover salt in the mine, and therefore we could assume that if a small village could get access to coal, they could get access to salt at the same time.