Suzhou is a city of silk in China. There are many silk factories in Suzhou. In the past, when reeling silk, it was very important for the workers to skillfully grasp the correct way to change the boiling water, as this would ensure the quality of the reeled silk. The women who did reeling had to check on the color of the boiling water from time to time. When there was a slight change, they had to replace one-third of the original with clean hot water, and continue the process. If the water color was the same all the time, the silk color was also always the same. To keep the reeling water clean, the silkworm breeders put a copper coin on the bottom of the cauldron. The criterion for the clarity of the water was decided by whether the coin could be seen clearly. This was the simple method the local people created and perfected over the years. Modern reeling is now done by machinery.