Fiberglass really is made of glass similar to that in windows or kitchen drinking glasses. To manufacture fiberglass, glass is heated until molten, then forced through superfine holes. This creates glass filaments that are extremely thin—so thin, in fact, that they’re best measured in microns. These flexible filament threads can be used in several applications: They can be woven into larger swatches of material or left in a somewhat less structured form used for the more familiar puffy texture used for insulation or soundproofing. The final application is dependent on the length of the extruded strands (longer or shorter) and the quality of the fiberglass. For some applications, it’s important that the glass fibers have fewer impurities, however, this involves additional steps in the manufacturing process.