The Mekhilta draws a vivid contrast between human construction and divine architecture. A human being builds a roof out of wood, earth, and stones, solid materials that resist gravity and keep out the rain. But the Holy One Blessed be He roofs His world with water. The proof comes from Psalms: "He roofs His upper chambers with water" (Psalms 104:3).
The image is staggering when you pause to consider it. Water flows downward. It is the least stable building material imaginable. You cannot stack it, nail it, or make it hold a shape. And yet God uses it to construct the ceiling of the cosmos. The waters above the firmament, described in the creation narrative of Genesis, are held in place by nothing but divine will. Where human engineers need rigid materials and careful calculations, God builds with the very substance that defies structure.
The Mekhilta calls this difference "the measure of flesh and blood" versus the measure of the Creator. The phrase appears throughout rabbinic literature as a way of marking the infinite gap between human capability and divine power. Humans work within the laws of nature. God sets those laws and then transcends them at will. A roof of water is impossible by every standard of physics and architecture. But the Mekhilta insists it is real, that the upper chambers of heaven are constructed from the very element that, in human hands, would bring any building crashing to the ground. What is weakness in human hands becomes strength in the hands of the Almighty.