When boiling water, the kettle makes a "humming" sound, but the water is not yet boiled. It is only when the sound changes to a "gurgling" and is not as loud as the initial "humming" that the water is truly boiled. Furthermore, whether the water is boiled can be determined by splashing a little on the ground. Boiled water makes a "puffing" sound when splashed, while unboiled water makes a "popping" sound. People have learned a lesson from this—boiled water makes no sound, while water that makes a sound is not yet boiled.
In addition, a red-hot kettle should not have cold water poured into it immediately, as it is very likely to crack. The correct method is to let the kettle cool down slowly before pouring in cold water.