Eggs aren’t exactly famed for strength. But have you ever considered what the thin egg wall was designed to hold up—the weight of full-grown mother chicken! A simple experiment will help you to see the secret behind the eggshell’s amazing strength.
An architectural marvel is sitting innocuously in your refrigerator. Other manmade structures, such as the exotic Taj Mahal or modern high-tech football stadiums, have borrowed the same exceptional design. But while those buildings demanded enormous amounts of human labor and money to construct, you simply picked up your masterpieces at the marketplace—a dozen of them, to be exact.
Opening the egg carton, you may not recognize the contents’ structural excellence. After all, eggs have a reputation for breaking easily. At the store we examine each carton to make sure nothing has oozed out, and at home one or two whacks are sufficient to open the shell.
But there’s more to eggs than omelets and cakes. If you’ve ever been tasked with plucking eggs from startled hens, you’ve uncovered the first evidence that these oblong spheres aren’t so fragile. After all, they absorb the weight of the chickens that laid them.
You can test this strength for yourself. Put on some gloves, grab an egg, and squeeze the rounded ends between your palms. Done correctly, the thin-shelled wonder won’t break. If you squeeze the flat sides, however, your fingers will plunge into a squishy mess.
Companies pack eggs with the pointy ends up to capitalize on the strength of their curved ends. The ancient Romans employed this same egg-shape in their aqueducts, bridges, and enormous amphitheaters. But you might be more familiar with its common names: the arch or the dome (which is essentially an arch rotated around its axis).
Eggshells get some of their strength from their unique mix of hard crystals (calcium carbonate) that are held together by flexible organic material (called a protein matrix). But much of the strength of the egg comes from the shape itself.
When a heavy weight presses down on an arch or dome, the force moves uniformly down the sides toward the base. The lack of angles or corners means that no one spot must handle the whole load. So hens can sit on their eggs with the arched end pointed up because the weight gets distributed evenly. The same is true when your hands push on the ends.
For this reason, arches and domes pack an architectural punch. They can enclose a great deal of space without needing internal supports, and they can span large distances without collapsing under the strain. (Try the experiment below.)
God built amazing strength into the eggshell’s shape to resist pressure from above—and yet it’s so delicate that a chick can peck its way out, and cooks can easily crack it open. Think about that the next time you pull a lowly egg carton from the refrigerator. There’s more inside than you might think!
Because weight is distributed evenly, the dome shape of the eggshell can support a significant load. Try this simple experiment to test its toughness. (Adults should supervise the experiment and perform all cutting.)
(A) Wrap tape around the middle of each egg. (The tape both prevents the eggshell from cracking while you cut it and helps to keep the edges smooth.) Poke a fairly large hole in the pointed (smaller) end of the eggs.
(B) Drain the contents of the eggs.
(C) Insert the scissors into the hole you made, and then carefully cut up toward the middle of the egg. Now make a horizontal cut around the middle of the eggshell. Discard the smaller end. Make sure that the edge of the remaining shell is as straight as possible and that it has no cracks. (Don’t peel off the tape because that could rip the shell.) Note: Make sure all the eggshell halves are as close in height as possible.
(D) Smooth the towel out on a flat surface. Place four eggshells on the towel, with the curved end up. Form a rectangular shape that’s slightly smaller than your largest book.
(E) Starting with the largest book, stack one at a time on your eggshells until one collapses.
Weigh the books to see how much weight your eggshells held. (A boy or girl weighing up to 40 pounds [18 kg] could actually stand on the shells without breaking them, if the eggs were cut perfectly and the child’s weight were evenly distributed on the shells!)
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