Previous research has documented a gap in students' understanding of gas behavior between the algorithmic-macroscopic level and the conceptual-microscopic level. A coherent understanding of both levels is needed to appreciate the difference in properties of different gases, which is not manifest in the ideal gas law. A demonstration that concurrently exposes both the difference and similarity in the behavior of two different gases was designed for this purpose. Instructions for performing the demonstration in an interactive manner are given. The theory of "knowledge in pieces" is used to analyze students' active participation and justify the proposed delivery method. (Contains 4 notes, 2 tables and 2 figures.)