The fact that you can't un-break an egg is a common example of the law of increasing entropy.
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“It was so hot today,” goes the old saying, “the chickens laid hardboiled eggs.”
“Hah,” says the rejoinder. “That’s nothing. It was hot enough here to fry an egg on the sidewalk.” So where did the idea originate? According to the Library of Congress, the phrase first appeared in print in the June 11, 1899, edition of The Atlanta Journal-Constitution newspaper. You can also find a photo of two women frying an egg on a wall in Washington, D.C., from June 14, 1929.
Whatever the saying’s origin, the idiomatic egg sure has legs. Every July 4, the town of Oatman, Ariz., on Route 66 holds its Solar Egg Frying Contest to see who can cook the protein-packed ovoids the quickest using solar power. Contestants may use any contraption they like – popular choices include hot boxes, magnifying lenses, pie tins, pans and the occasional compact disc – and ingredients such as bacon and potatoes, as long as the sun is their only source of heat. Some succeed; many don’t.
But this myth, at heart, describes a simpler and more elegant arrangement: Egg + Sidewalk + Hot Day = Brunch. In other words, stripped of every contraption, can a sidewalk serve as a skillet?
Frying an egg involves more than a flicker of heat and a lot of patience. It requires denaturing an egg’s proteins – that is, changing their molecular structure. Denaturation accounts for why egg whites change from transparent to opaque when you cook them, and the process cannot kick off below around 158 degrees F (70 C).
Let’s stack the deck in favor of the myth. The hottest temperature ever recorded on Earth was 134 degrees Fahrenheit (56.7 degrees Celsius) was actually recorded in Death Valley on July 10, 1913 (declared so in September, 2012). That’s not hot enough to cook an egg, but it’s also not as far off as it sounds. Sidewalks, after all, often blaze hotter than the surrounding air because they store heat and release it in an every-changing energy balance. The daylong absorption and release of heat by various materials helps explain why the mercury climbs higher in the afternoon than at noontime.
Would this extra heat be enough to fry an egg on a 134-degree day? Probably not. The handful of extra degrees contributed by stored heat in concrete is simply not up to the task. Unlike blacktop, light-colored sidewalks reflect more energy than they absorb. Moreover, concrete is a poor heat conductor, so only a fraction of its warmth would transfer into the egg.
Frying an egg on the sidewalk requires some sort of solar assist. Check out the ideas in this video.