Answer:
It has to have a whistle device on the spout. When boiling occurs, water vapour displaces air above the water which has to escape through the whistle hence activating the required vibrations. Thereafter, the continuing supply of steam does the same job until the cook involves turns off the heat. The whistle is not made to exacting scientific standards and there is no doubt a range of pressures above which it will work it. It would be interesting to know how you observed the water boiling without using a transparent kettle.
Explanation:
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