One of you writes the instructions: at most 5 lines, in the pen language a child reads in DoodleForge (forward(60) moves and draws, turn(90) turns right, a for loop repeats). The other draws them on paper, by hand, without seeing anything else. Then let the pen draw the same lines and put the two side by side. Where they differ, read the line again: was it the instruction, or the reading?
The other person draws on paper, line by line. A pencil and any sheet will do.
The pen only draws after a person has.
From 1968 the American artist Sol LeWitt made wall drawings that other people drew. He wrote the instructions; assistants, and later anyone, drew them on the wall, so each drawing came out a little different every time it was made. In “Paragraphs on Conceptual Art” (1967) he wrote: “The idea becomes a machine that makes the art.” Here the idea is your five lines, and there are two machines: a person, and a pen that does exactly what it is told.
Sources: S. LeWitt, “Paragraphs on Conceptual Art”, Artforum 5:10 (June 1967). The children’s side is DoodleForge on Spark & Anvil, where a child reads a pen program and guesses the drawing before it is drawn.