Prior to this exercise, I introduce some fundamental concepts regarding systematics and phylogenetic reconstruction. I do it early in the semester because I like to have the students thinking about these things while we are discussing the major fossil groups later in the semester. The exercise is simple: students construct a phenogram using a collection of objects -- could be fossils, or toy animals, or whatever. I use about ten things. At first, students think it'll be easy. They soon realize how difficult it is to identify characters that group some taxa but exclude others. Obviously, the particular taxa you choose can make the exercise more or less difficult. I should also mention that I require the students to produce a dendrogram with strictly dichotomous branching. I explicitly refer to it as a phenogram because I allow students to use whatever characters they wish, and it isn't even necessary that they be able to recognize the organisms. Inevitably, this exercise leads to discussions about the importance of phylogenetically informative characters.