system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Can someone please explain to me how exactly a 3-D printer works? My mind simply cannot grasp the concept of a printer that can make an actual gun that can shoot real bullets.

Edit:  Thanks for all of the useful information.  The concept is much clearer, even if I still don't totally get it.  And thanks to those who spoke up about not understanding as well.  I don't feel quite so old and out of touch.  :)
gpt
First imagine if we could put a gun onto a deli slicer.  Then start making slices of the gun that are very thin.  1mm or less in thickness.  The 3D printer works in the reverse of this.

It will take something that is in liquid form, but will solidify when lowered to room temperature and exposed to air (like liquid latex, cyanoacrilate, heated up plastic).

So it lays down the first slice.  Then the next slice on top of it, and so forth until you have the gun (this is greatly simplified, but illustrates the concept).

To shoot real bullets isn't terribly difficult.  It just requires a hammer to strike a firing pin.  You could make one out of papier mache as long as you had a hammer (not taking accuracy or rifling into consideration)