Scientists use a tool called a key, sometimes a "dichotomous key," to categorize and identify different varieties of things. There are keys for identifying trees, keys for identifying insects, keys for identifying birds. Whether online or in book form, the keys are essentially a system of "Twenty Questions." For example, if we wanted to identify a tree from one of these keys, perhaps the first question would be "Is the tree deciduous or coniferous (broad leaves or needles)?" If the tree fit the first category, the key might instruct us to turn to a specific page to answer more questions.
