A rat receives food pellets for pressing a lever, but only on unpredictable occasions. This BEST illustrates

i'd say that the moon rock is about 2.2 billion years old.

the elements of art are the visual tools that the artist uses to create a composition. these are line, shape, color, value, form, texture, and space.

the principles of art and design are balance, contrast, emphasis, movement, pattern, rhythm, and unity/variety.