If you had access to a 3D printer and could create only one thing out of plastic, from scratch, what would you make? An implantable human ear replacement? A filter for pumping clean water in thirsty third-world nations? What about forming something fun and whimsical like, say, an acoustic guitar? Or, would you take the tunnel of least resistance, and the road of the lowest common human morality, and choose to print a plastic gun for killing people?