Reasons for waitankung's popularity are not hard to find: It's easy to learn, it doesn't require any special equipment, the time to practice is short (a half an hour a day will do) and, equally important, there's no fee.
一般認為，外丹功能在短短七年內迅速發展，有幾個因素：易學、不佔地方、練習時間短（半小時即可），也不需要特別配備。同樣重要的，是不必繳學費。