When boiling water, the kettle makes a "humming" sound, but the water is not yet boiling. The water is truly boiling when it makes a "gurgling" sound, which is not as loud as the initial "humming". Furthermore, whether the water is boiled can also be determined by splashing a little on the ground. Boiled water makes a "puff" sound when splashed, while unboiled water makes a "pop" sound. People have thus learned a principle—boiling water is silent, while silent water is not yet boiling.
Additionally, a red-hot kettle should not have cold water poured into it immediately. If cold water is poured in, the kettle is very likely to crack. The correct method is to let the kettle cool down slowly before adding cold water.