Space.

There are about a billion ways to demonstrate and perceive space in art.  Oneof the coolest ways this can be done is"chiaroscuro"... and not just because it's a foreign word.  Chiaroscuro "creates the illusion of space" with lightness and darkness (Launching the Imagination, p. 32).  Essentially, when someone uses chiaroscuro, they will use a lighting effect that resembles a natural scene.  ...When we were children, we used to draw in a two-dimensional state where the sun pops out of the corner of the page, and the other things in the picture lie equally flat on the plane,  Eventually, we may have tried to draw three-dimentionally, but it wouldn't have been very effective if there weren't any shadows in our color-crayon explorations.  Applying chiaroscuro to the idea of the sunshine in the corner of our lovely childhood drawing, we should have made everything in the drawing brighter where it faced the sun, and everything on the opposite side much darker, to create the illusion of multi-dimensiality past numero dos.  This is the example the book uses (which is flawless), Judith and Her Maidservant with the Head of Holofernes by Artemesia Gentileschi: