We've watched people walking barefoot on hot coals without burning their feet, but what about walking barefoot on ... eggs without breaking them? Actually, you can walk on eggs without breaking them if you distribute your weight carefully. Don't believe me? Watch this video:
Eggs are built to hold the weight of a full-grown bird sitting on them, but to easily break open so a tiny chick can get out. This is possible because the incubating bird's weight is distributed evenly over the outside of the egg whilst the chick pecks at a small place inside the egg when it hatches. This is how the incubating bird can sit on her clutch of eggs without crushing them, yet a chick can escape its eggshell when it is ready.
In the lab portion of my ornithology course, we were challenged to crush a chicken egg with one hand by wrapping our hand around the egg and squeezing it. No one could do it because the pressure of the squeezing hand was distributed over the entire eggshell. But of course, it was quite easy to break that same egg by putting pressure on a small portion of the shell -- by banging on the shell with the blade of a knife, for example.
The structure of a three-dimensional arch, such as what you see in stone entryways, bridges and cathedrals, is quite similar to the structure of the pointed end of an egg. Similar to squeezing an egg in your hand, this arch is very strong, holding up hundreds of tons of material.
.. .. .. .. .. .. .. .. .. .. .. ..