3D printers can create objects with internal, movable parts. Instead of having to print individual parts and have a person assemble them, a 3D printer can print the object already assembled. Of course, a 3D printer can also print individual parts or replacement parts. In fact, some 3D printers can print a substantial number of their own parts, essentially allowing them to self-replicate. 3D printing starts with a blueprint, usually one created with a computer aided design (CAD) program running on a desktop computer. This is a virtual 3D model of an object. CAD programs are widely used today by designers, engineers, and architects to imagine physical objects before they are created in the real world. The CAD design process replaces the need to design physical prototypes out of malleable material such as clay or styrofoam. A designer uses the CAD program to create the model, which is then saved as a file. Much as a word processer is superior to a typewriter because it allows a writer to add, delete, and edit text freely, a CAD program allows a designer to manipulate a design as she sees fit. Alternatively, a 3D scanner can create a CAD design by scanning an existing object. Just as a flatbed scanner can create a digital file of a drawing on a piece of paper, a 3D scanner can create a digital file of a physical object. No matter how it is created, once the CAD design exists it can be widely distributed just