Keystones
The arch is an engineering marvel. It requires a set of objects to transfer force through them in an equal amount so that each side holds the other and they both hold the entire structure in the embrace of gravity. At the center of this exchange is the keystone, an oddly-shaped bit that allows both arms of the arch to press equally against each other. I think back to times when I have felt like the stones: pressed from behind and pushing forward without any change in position
