Yak Butter,refined from the milk of cattles and goats,is the daily food of Tibetans.It is intriguing for Tibetans to refine yak butter.As milk segregators are not widely used in the pasturing area in Tibet,Tibetan people usually refine milk in old way.Tibetan women pour the heated milk into a big wooden bucket called "Xue Dong",then whip forcibly up and down for hundreds of times to segregate grease from water.Step by step,a tier of somthing light yellow will surface, subsequently ladle the floating and put it into a leather bag to cool it.In this way,the refinement of yak butter ends.Yak butter has very high value of nutrition. Tibetans usually eat fruits and vegetables little,especially in pasturing area,the daily required heat energy is supplied by yak butter besides meats.