What Makes the Kettle Whistle?
The kettle whistles only when the water boils and the steam or gas which is the form the water turns into when boiling is trying to escape through the spout of the kettle. You see, when the water starts boiling, the inside of the kettle is at once filled with steam and more is coming out of the water all the time. This steam must get out some way, so it rushes for the spout of the kettle, and because so much of it is trying to get out of a comparatively small opening at once there is quite a pressure and this results in making the whistle out of the spout of the kettle. It is just the same process as when you whistle yourself. To whistle you fill your mouth with air and force it out through your lips, which you have closed excepting for a small opening, by the pressure you can bring to bear with the roof and sides of your mouth, and if you have learned to make your lips into the proper shape and apply the pressure steadily you can sound a very long note and make different notes by making the opening in your lips large or small. The kettle spout has only one size of opening so the sound is practically the same at all times though louder at sometimes than at others. This is caused by the varying pressure at which the steam in the kettle is being forced out.