When boiling water, the water in the kettle makes a "buzzing" sound, but the water is not actually boiling. The water is truly boiling only when the sound becomes a "gurgling" and is not as loud as the initial "buzzing" sound. Furthermore, whether the water is boiled can also be determined by splashing a little on the ground. Boiled water splashed on the ground makes a "puff" sound, while unboiled water makes a "pop" sound. People have thus learned a principle—boiled water is silent, while boiling water is noisy.
In addition, a red-hot kettle cannot have cold water poured into it immediately. If cold water is poured in, the kettle is very likely to crack and explode. The correct method is to let the kettle cool down slowly before pouring in cold water.