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If you build four walls to create a room, you still need a way to cover it. The simplest option is to lay beams across the top and cover them with planks. But wood rots, and it burns. So you decide to build your roof from stone. But stone is heavy, and a flat stone beam cracks under its own weight if the span is too wide. So instead of fighting gravity, you work with it. You arrange wedge-shaped stones in a curve so each one leans against its neighbor, channeling the weight sideways and dow