Fiberglass really is made of glass, similar to windows or the drinking glasses in the kitchen. The glass is heated until it is molten, then it is forced through superfine holes, creating glass filaments that are very thin – so thin they are better measured in microns. These threads can then be woven into larger swatches of material or left in the somewhat less structured although more familiar puffy substance used for insulation or soundproofing. This will depend on whether the extruded strands were made longer or shorter, and the quality of the fiberglass. For some applications, it is important for the glass fibers to have fewer impurities, which involves additional steps in the manufacturing process.