"Working models are not required for patents, merely drawings and descriptions. But I needed good models, models that would work perfectly and anybody could demonstrate, because these models were going to have to sell themselves, show by their very practicality and by the evident economy designed into them for their eventual production engineering that they would not only work but would be a good investment—the patent office is stuffed with things that work but are worthless commercially." - Heinlein, The Door Into Summer (1957)