Q. Why does a kettle sing more when it is set on the side of a fire, than when it is set in the midst of the fire?
A. When the kettle is set on the hob to boil, the heat is applied very partially: one side is hotter than the other, and therefore the steam is more entangled.
Q. Why does a kettle sing, when the boiling water begins to cool again?
A. Because the upper surface cools first; and the steam (still rising from the lower parts of the kettle) is again entangled, and escapes fitfully.
Q. Why does boiling water swell?
A. Water (like air) expands by heat. The heat of the fire drives the particles of water further apart from each other; and (as they are not packed so closely together) they take up more room; or (in other words) the water swells.
Q. What is meant when it is said, “that heat drives the particles of water further apart from each other.”
A. Water is composed of little globules, like very small grains of sand; the heat drives these particles away from each other; and (as they then require more room) the water swells.
Q. Why does boiling water bubble?