I once sold encyclopaedias. I had a sample volume, richly bound, with examples of articles from different sections of the complete set, and a fold-out, life-size illustration of the whole set to lay out before prospective customers. I could talk about the quality of the product, the essential and comprehensive nature of the contents, and the long-term value of owning the books. Had I been asked in-depth questions about the articles, however, I could not have answered. I was expert in the product, not in the content.