Is the use of value to create the illusion of depth.
a. shadowing
c. modeling
b. foreshortening
d. sfumato

An illusion is a distortion of senses where a person has the visual ability to perceive an object and modelling is use to create the illusion of depth. This is done by reducing the size of the object, placing it in front or behind another object or by shading.

Here is a picture of a turtle, just an idea though