Science Puzzle
Why Is an Egg So Hard to Crush?
Crack an egg on the edge of a bowl and the shell gives way with the lightest tap. Yet hold an egg end to end in the palm of your hand and squeeze, pressing evenly on its tip and its base, and most people can't break it at all. (Take any rings off first and try it over the sink.)
How can the same thin shell be so weak one way and so strong the other?
The Answer
Each end of an egg is a dome, and a dome pushed evenly from outside spreads the force all the way round its curve.
When you press on the tip of an egg, the curved shell carries the push sideways and downwards through the whole shell, squeezing the material together rather than bending it. Shell is made mostly of calcium carbonate, which is strong when it's squeezed but weak when it's bent or stretched. A dome uses that strength perfectly: every bit of shell takes a small share of the load and nothing has to bend.
Tapping the egg on the rim of a bowl is completely different. The hard edge pushes on one tiny spot. The shell there has to bend inwards, its inner surface is stretched, and a crack starts, then runs across the shell. A thin shell has almost no defence against a sharp point.
Builders have used the same trick for thousands of years. Domes and arches can be made of stone or brick, materials that are weak when bent, because the curve turns the weight above into a squeeze along the arch. The dome of the Pantheon in Rome, built nearly 1,900 years ago from unreinforced concrete, is still standing.
The principle: Arches and domes. A curved shell spreads a load around its curve as compression, so a thin, brittle material can bear a large even force, though it stays weak against a sharp blow at one point.