Three-Boiling Diagram
    Ancient people were very particular about heating water called "Tang Hou".
    As early as the Tang Dynasty, Lu Yu summed up a three-boiling method of heating water in Chapter Five "Brewing" of the "Tea Classic". When the water first boils, an image appears like the eyes of fishes are on the surface; this is the first boiling. Then appears something like a spring rushing forth and a string of pearls at the side of the caldron; this is the second boiling. Then along come the waves and breakers; this is the third boiling.
    Lu Yu considered that water of the third boiling was over-boiled and should not be drunk. He pointed out that seasonings should be put in during the first boiling and tea be put in during the second boiling.

    During the Song Dynasty, however, a kettle with a small neck was used to heat water instead of the uncovered caldron used in Tang Dynasty, as a requirement of  "dian cha" (infusing tea). Then it was difficult to identify the "Hou Tang" with naked eyes. So Cai Xiang said: "Hou Tang was the most difficult". Therefore, another method and standard to identify "Hou Tang" was proposed, i.e. identifying by hearing the sound of boiling water.
Song's people considered that when the water first boiled, noise like insects chirping could be heard. This was the first boiling. When the sound was like a cart rumbling, this was the second boiling. When the water sound was like the singing of the wind in the pines or the murmuring stream, this was the third boiling. Then it was time to take up the kettle and pour the boiling water into the cups containing tea powder. The earlier three boiling was required in the "Hou Tang" because the infusing method of Song Dynasty was different from the cooking method of Tang Dynasty.