On Food and Cooking

and maintain a particular cooking
temperature, and reproduce the same
temperature reliably. Thermostats,
thermometers, and our senses are all fallible.
So one of the great advantages of water as a
cooking medium is that its boiling point is
constant — 212ºF/100ºC at sea level — and
it’s instantly recognizable. The sure sign of
boiling water is bubbling. Why? When the
water in a pan is heated near boiling,
molecules at the bottom, where the pan is
hottest, vaporize and become steam, and form
regions that are less dense than the
surrounding liquid. (The small bubbles that
form very early on are pockets of air that had
been dissolved in the cold water but became
less soluble as the temperature rose.) Because
all the pan heat at the boil goes into
vaporizing the liquid water, the temperature
of the water itself stays the same (p. 816). It’s
only slightly higher at a full, rolling boil than
in a gently bubbling pot, and will not get any