Peek through the inspection windows of the nearly 100 three-dimensional (3D) printers quietly making things at RedEye, a company based in Minnesota, and you can catch a glimpse of how factories will work in the future. It is not simply that the machines run day and night【C1】______by just a handful of technicians.【C2】______it is what they are making that shows how this revolutionary production process is【C3】______the manufacturing mainstream. 3D printers make things by building them up, a layer at a time,【C4】______a particular material, rather than【C5】______it by cutting, drilling or machining—which is why the process is also called additive manufacturing. There are many ways in which this can be done, and with only a【C6】______adjustment of software each item can be different,【C7】______the need for costly retooling of machines. This has made 3D printing a【C8】______way to make one-off items, especially prototype parts, mock-ups, small mechanical【C9】______and craft items. And that is about all that 3D printers are good for,【C10】______the doubters. Chief among them is Terry Gou, the boss of Foxconn. He thinks 3D printing is just "a trick" without any【C11】______value in the manufacture of real finished goods, and he has vowed to start spelling his name backwards if【C12】______wrong. Mr. Gou is right about one thing: additive manufacturing is not about to replace mass manufacturing.【C13】______the technology is improving, the finish and durability of some printed items can still【C14】______what producers require. And nor can 3D printers produce millions of【C15】______parts at low cost, as mass-production lines can.【C16】______3D printers have their【C17】______which is why they are starting to be used by some of the world"s biggest manufacturers, such as Airbus, Boeing, GE, Ford and Siemens. The market for 3D printers and【C18】______is small, but growing fast. Last year it was worth $2.2 billion worldwide, up 29% from 2011. As producers become more【C19】______with the technology, they are moving from prototypes to final【C20】______.