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Although the glass in your windows may look as clear as a crystal of quartz or sapphire, its structure is radically different. Crystals have a regular, rigid structure on the atomic scale, while glasses at room temperature are messy collections of molecules that flow like an extremely viscous liquid. As glass that has been heated in an oven like the one shown here cools, its viscosity increases dramatically, leaving it stuck far from its ground state in what appears to be a stable solid form. (Unit: 9)