After a heated (but to reach 70-80 ℃) of milk cooling solidification, the surface layer of Naipi will appear, then we know that this film is a layer of fat. And we re-heating a milk drink and found it only a little Naipi . We then had another bottle of water to dilute the pure milk, heated Naipi only a little bit, not that you can use this method to test the purity of milk.