A 3D pen is basically a colored glue gun.
I often describe 3D printers as essentially a glue gun on the end of a robotic arm putting down layers one at a time.
@Alex_Wiebe “glue gun on rails” is the wording I use. 
In that case, a 3d printer is basically a CNC glue gun. Anyone ever actually tried a glue gun on a robot arm or rails?
@mark_warlick yes, the first 3d printer was a glue gun and a bunch of mechano part.
I guess that also means that my glue gun uses 1/4 inch diameter filament