Avogadro's conception of the structure of gases was not widely accepted by his contemporaries--probably because the hypothesis was not supported by direct evidence. This problem is rarely addressed in schools. This article discusses the difficulties that accompany the acceptance of new ideas. Such difficulties may be associated with the ways in which scientists reason. Furthermore, we have developed a simple experiment in which these complex matters can be introduced to students. Specifically, we put students into the situation that was faced by chemists 200 years ago. This article aims to discuss (1) why Avogadro's ideas were not accepted by his contemporaries, and (2) how these ideas could be used to teach chemistry in a meaningful way.