Create a key to trees sampled from around the schoolyard. Use only features that are clearly visible and consistent between the different specimens you have that are from the same species. Make the key as simple (i.e., the fewest steps) and as user-friendly (i.e., the fewest new terms or hard choices) as possible. This is a fun way to gain direct familiarity with the idea of a dichotomous (branching in two parts) key. You can create keys to lots of everyday things - children in your classroom, types of silverware, seeds, toys, art supplies, tools, etc. Eventually, students can create and continue build and refine their own keys to the organisms they observe and describe in their schoolyard studies - leaves, branches, fruits, insect, etc.