"3D printing refers to various processes used to synthesize a three-dimensional object. In 3D printing, successive layers of material are formed to create an object. These objects can be of almost any shape or geometry and are produced from a 3D model or other electronic data source." - Wikipedia
But how do you teach that to a five year old kid so that he or she could use a 3D printer? Well, just like any other thing you want to teach - Let them experience it first hand and make something.
When I installed the printers in the kindergarten, the kids looked at them with wonder. I told them that we are going to print our own toys, but could clearly notice they didn't understand the meaning of it. At one point a kid asked me if she could bring a doll to put in the printer.
I had to start from the very beginning and introduced the kindergarten to Tinkercad, a browser-based 3D solid modelling tool for rapid prototyping known for its entry-level ease of use.
It has an intuitive interface and is a great tool to demonstrate the meaning of 3D. At first I would pick and arrange a few bodies and show how they appear from different points of view. A sphere would be seen the same from all angles, but a cylinder could appear as a square or a circle. The arrangement of the bodies in the space also affects how a body might hide behind another. The kids got the hang of it in a few days. They played with the software and we saved their projects just like we did with their drawings on Paint.
To demonstrate the concept of layers in a 3D printing process, I developed an easy activity in which the kids take an active part in.
I chose three identical cylinders and placed them on the printer bed in different orientations. We talked about the way the three are positioned and I asked them to draw the three shapes as they see them on the screen. It was interesting to see it through their eyes, most of them weren't able to draw it perfectly, but the exercise was good. I then asked them to choose a color for the first layer and started printing. At 25% I paused the printer and asked them to draw the partial print they see.
I let them choose and switch to another color, resume the print for another 25% and pause. During a print, the bodies are "open", the outline of the layers are clear and the infill is visible. Again I let them draw what they saw, this time with two colors. We kept on through the day until all four layers were finished and we had three cylinders with the same dimensions and different color distributions.
In this exercise the children get to be a part of converting a digital shape into a solid object. I think the best way to understand the concept and logic behind 3D printing is to experiment with it yourself.
Here are some toys the kids chose from Thingiverse. I also designed 3D name signs and each kid chose the colors for base and letters.