Carl and I both smiled indulgently. Alan was not a science geek, despite a background in software development. I gestured with my hands as I described the idea. "You’ve seen 3D printers, right? Printing things like plastic parts, medical prosthetics, and toys?" At his nod, I continued, "So take it to the next level. Have them able to deliver any element, one atom at a time, according to a design. You could, in principle, print literally anything solid."