I did some art a few years ago for a new think tank based in Chicago. The name was Greenhouse, so I set to work thinking of every green metaphor and every way I could incorporate greenness into illustrations of other things. Simplicity of line helps a lot. The simpler the drawing the more metaphor it can convey, because concrete art emphasizes the object and its qualities. Simpler art gets the viewers or readers to ask themselves: what else does this mean? Simple, unelaborated line can also suggest other things, other topics, other meanings. Is this simple to do? Not really. A simple drawing often reaches that nice evocative simplicity after pages and pages of other versions have wound up on the floor.