In this presentation, we investigate the “impossible examples” we find in microeconomics textbooks. The terminology “impossible examples” designates either examples describing an « impossible world » or examples which describe real world but are partly inconsistent with the very concept or hypothesis they exemplify. Focusing our analysis on the presentation of the « competitive » firm, and on the examples given to illustrate the production process, we ask the question “What can students possibly understand and learn through these impossible examples?”