Humans rely on visual cues (form, scale, color, value, etc.) when interpreting sensory visual data from the environment. Our environment is regular and predictable enough for us (through 'survival-of-fittest' we adapted specifically to this environment and not some other one) that over time we have become accustomed to successfully attributing certain meanings/outcomes to certain cues. When viewing a distant object moving in space, for example, (person, ball, airplane, lion charging, etc.) we use visual cues, such as change in size (scale), change in silhouette/negative space (shape or form), change in reflected light (value), to interpret what we are seeing. Through these cues we gain an accurate sense of depth perception and 3D space.
We are so well adapted and consistent in our response to certain universal cues, that we can 'fall prey' to illusion, if the same response (perceptual event) to the visual information can be triggered in the absence of the genuine real-world stimulus. As animator-artists, we can exploit this tendency when attempting to create the "illusion of life". Animation involves the careful study of life in action (reference), then the interpretation/processing of that careful study into a series of 2D drawings (drawings that convey shape transformation over time as each new drawing in the series is changed slightly from the previous). As animators attempt to replicate what is experienced in reality, when drawing an animation, they exaggerate these observations. Through exaggerating the observations, the action will be more clearly conveyed to an audience.
