"to scrape wind", as in Vietnamese culture "catching a cold" or fever is often referred to as trúng gió, "to catch wind". The origin of this term is the Shang Han Lun, a ~220 CE Chinese Medical text on cold induced disease. Some methods use oil balm and a coin to apply pressure to the skin. Others use a boiled egg with a coin inserted in the middle of the yolk. The egg is wrapped in a piece of cloth and rubbed over the forehead (in the case of a fever) and other areas of skin. After the rubbing, when the coin is removed from the egg, it will appear black.