Q: My wife and I disagree on whether a pot of water will boil sooner if you keep the lid on. She says it will, because without the lid a lot of heat would be lost. I say that it will take longer to boil, because the lid increases the pressure and raises the point at which water will boil, as in a pressure cooker. Who's right?

A: Your wife wins, although you do have a point.

As a pot of water is heated and its temperature goes up, more and more water vapor is produced above the surface. That's because more and more of the surface molecules gain enough energy to leap off into the air. Note that I'm talking here about water vapor, not steam. Steam doesn't occur until the water is close to boiling, when there is so much water vapor in the air that some of it condenses out into tiny droplets that stay suspended in the air because they are too small to fall out. Steam can be seen, but water vapor is an invisible gas. (You can't see humidity, can you?)

Anyway, as the water is heated, the increasing amount of water vapor carries off an increasing amount of energy that could otherwise go into raising the water's temperature. And the closer the water gets to the boiling temperature, the more energy it loses in the form of the hotter vapor molecules, so the more important it becomes not to lose them. A pot lid partially blocks the loss of these vapor molecules. The tighter the lid, the more of those hot vapor molecules are retained in the pot and the sooner the water will boil.

Your point, that a lid increases the pressure inside the pot as in a pressure cooker, thereby raising the boiling point and delaying the actual boiling, is correct in theory but insignificant in reality. Even a hefty one-pound lid on a ten-inch pot would raise the pressure inside by less than a tenth of a percent, which would in turn raise the boiling point by only four hundredths of a degree Fahrenheit. You could probably delay the boiling longer by watching the pot.

Eggs and sidewalks, revisited

Following my recent column on whether it ever gets hot enough to fry an egg on the sidewalk, several readers informed me that they have seen it done.

C. Paul Mendez of Glenmont, Md., says that in a World War II German newsreel he saw two Afrika Korps soldiers fry an egg on the fender of a tank. That's not exactly a sidewalk, but I observed during my own experiments in Texas that automobile surfaces do get hotter than sidewalks. (Austin's streets were mercifully free of tanks, although some RVs came close.) "They cleaned off a spot," Mendez recalls, "poured on a little oil, spread it around and then broke two eggs onto the surface. The whites turned opaque just as quickly as they do in my frying pan."

Closer to home, as the TV news people say, Karen Campbell of Clifton, Va., reports that 10 years ago she and some young friends cooked an egg on a sidewalk in Tempe, Ariz., when the air temperature was 122 degrees. She didn't measure the temperature of the sidewalk. The highest recorded temperature in the U.S., incidentally, was 134 degrees at Greenland Ranch in Death Valley, Calif. (not too far from Tempe) on July 13, 1913. That was exceeded only by the 136-degree world record measured in El Azizia, Libya (not far from Mendez's tank) on September 13, 1922. But I doubt that either place had sidewalks.

"The egg came [straight] out of the refrigerator," Campbell writes. "We cracked it directly on the sidewalk and immediately the white started cooking. In less than 10 minutes the yolk broke . . . and spread out and the [whole] egg cooked. . . . We thought maybe it was a fluke that the yolk broke, so we tried another one and the yolk broke on that one, too, in about the same amount of time."

Now, of course, I have to explain why the yolks broke and spoiled the possibility of preparing sunny-side-up street snacks. I can only guess, but Campbell gives me a clue.

"We went back inside the house," she continues, "and a little while later my friend told us we'd better go clean up the eggs before her husband got home, so we went back outside. The eggs were completely dehydrated and broken into little pieces and there were a bunch of ants carrying off the pieces; we had nothing to clean up."

Aha! That's the answer: dehydration. In Arizona the humidity can be so low as to be almost nonexistent, so liquids will evaporate and dry up in a flash. What must have happened is that the surface of the egg yolk quickly dried out, became brittle and cracked open, spilling its still-liquid contents. Eventually, the whole egg schmear dried out and cracked into small platelets, like mud does in a dry lake bed or soil does during a severe drought. The platelets were just the right size for the happy ants to cart off to wherever it is that ants have their afternoon tea.

The wonderful thing about science is that it can even explain things that nobody really wants to know.

Robert L. Wolke is professor emeritus of chemistry at the University of Pittsburgh and the author of "What Einstein Didn't Know--Scientific Answers to Everyday Questions." Send your food or cooking questions to wolke@pop.pitt.edu.