Q. How do these two currents pass each other?
A. The hot ascending current passes close by the metal sides of the kettle; while the cold descending current passes down the centre.
Q. Why does boiling water bubble?
A. The bubbles are portions of steam (formed at the bottom of the vessel) which rise to the surface, and escape into the air.
Q. Why does a kettle run over, when the water boils?
A. As the heat insinuates itself between the particles of water, it drives them asunder; and (as the particles of water are driven apart from each other) the same vessel will no longer hold the expanded water, and some runs over.
Q. Why does a kettle sing, when it is about to boil?
A. Water contains a great deal of air, which (being expanded by the heat of the fire) escapes by fits through the spout of the kettle; which sings in the same way as a trumpet does, when a person blows in it.
Q. Why does water boil?