3D printing is still a niche industry, largely due to the costs associated with the machines and materials. But as those costs decrease, 3D printers are being used for a form of production that designers call "mass customization." 3D printers can quickly produce large amounts of products that have all been tweaked or modified in unique ways. For example, 500 people might order a certain kind of doll, but some may want black eyes and some may want purple, while others want may desire a certain nose or mouth. Because 3D printers build directly from the digital model instead of using a mold, end products can offer that kind of nuance at no additional cost to manufacturers or consumers. But with mass customization, there comes a new challenge: How is it possible to keep track of all those objects, each one the tiniest bit different? Researchers at Microsoft have developed an answer: a new method of marking objects without leaving a visual trace. The method involves creating objects with various internal gaps, or bubbles, within its body that form predetermined patterns. These patterns can then be observed using a Terahertz scanner, a device that has been used in airport security since 2007.