The Stone That Holds Everything

Stand in Rome and look up at the Colosseum's ancient arches — still standing after two thousand years of earthquakes, invasions, and relentless tourism. Engineers will tell you the secret isn't the massive stones you can see, but a single wedge-shaped stone at the very crown of each curve: the keystone. It's the last piece lowered into place during construction, and the moment it locks in, something remarkable happens — the entire arch becomes self-supporting.…