University of Southern California professor Berok Khoshnevis was troweling plaster onto the wall of his home when it occurred to him that a small trowel on the tip of a rapid-prototyping machine could remove the layered appearance, according to Paul Dvorak in Machine Design magazine. With funding from the National Science Foundation, Khoshnevis has developed this 3D extrusion system and now says it could be used to build concrete homes. Concrete would be extruded from a nozzle mounted on a large gantry and rails at the jobsite. As layers build up the robotic trowel would come back to smooth them and before you knew it, with little human intervention, the house would be complete. Exotic housing designs will be limited only by imagination—and gravity, professor Khoshnevis says.