The fired or sun-dried clay unit — hand-sized, standardized, and stackable — that is among the oldest manufactured building materials on earth, and whose mass production from Mesopotamia to the Indus Valley, from Roman cities to the industrial age, has always depended on the muscle of the people who made and laid it.
Pick up a brick. It weighs about two and a half kilograms. It fits in one hand. That size is not arbitrary: it is the largest unit a bricklayer can lift and place in one motion, thousands of times a day, without burning out by noon. That relationship between the human body and a fired clay rectangle is among the oldest engineering decisions in the world — made roughly eleven thousand years ago (around 9000 BCE), independently replicated across civilizations, and still holding. The brick is the most democratic building material ever invented: small enough for one person to handle, simple enough to be made from local soil, and modular enough that enough of them, stacked and bonded, can build a palace or a wall or a city. Almost nobody notices that the simplest thing in the built world is also one of the most consequential.