Note Footnote 3 — CHAPTER FOUR

All the extra heat you put into boiling water goes toward changing the water into steam; it cannot raise the water's temperature, because at the moment when water gets above the boiling point it ceases to be water and becomes steam. This steam takes up much more room than the water did, so it passes off into the air. You can tell when a teakettle boils by watching the spout to see when the steam3 pours forth from it in a strong, steady stream. If the steam took no more room than the water, it could stay in the kettle as easily as the water.