A. Water contains air; and (as the water is heated) the air is driven out, and raises a bubble in that part of the water which resists its escape.
Q. Why does a kettle sometimes boil over?
A. Liquids expand very much by heat; if, therefore, a kettle be filled with cold water, some of it must run over as soon as it is expanded by heat.
Q. But I have seen a kettle boil over, although it has not been filled full of water; how do you account for that?
A. If a fire be very fierce, the air is expelled so rapidly, that the bubbles are very numerous; and (towering one above the other) reach the top of the kettle, and fall over.
Q. Why is a pot, which is full to overflowing (while the water is boiling hot), nothing like full, when it has been taken off the fire for a short time?
A. When the water was swelled by boiling heat, it filled the pot even to overflowing; but as soon as the water is condensed by cold, it contracts again, and occupies a much less space.
Q. Why does the water of a kettle run out of the spout when it boils?
A. Because the steam cannot escape so fast as it is formed, and (being confined in the kettle) presses on the water with great power, and forces it out of the spout.