Dichotomous Key Activity “Odds and Ends”
Scientists use keys to help identify and classify plants and animals. By organizing specimens based on similar characteristics, scientists can also better understand how these species might be related to each other on an evolutionary level. Keys can come in many different formats. Some are used to identify organisms into larger categories, such as mammals and reptiles, and others are used to distinguish among closely related species.

A dichotomous key presents the user with a series of yes/no statements regarding distinct characteristics of the specimen. (“The animal has a backbone.” OR “The animal does not have a backbone.”) These statements are sometimes referred to as couplets. A couplet is essentially and either/or choice. The specimen is correctly identified when one makes the appropriate choice for each set of characteristics in a series of consecutive steps.

This activity introduces students to the use of a dichotomous key using everyday items. The goal of this activity is NOT to try to identify these objects, but rather to use a key correctly.

What you need:
- One Bag of ‘Odds and Ends’ for each group of 3 students
- One Key to Odds and Ends worksheet for each group

Each Bag of ‘Odds and Ends’ contains:
A small metal paper clip, a large metal paperclip, a plastic coated paperclip, a wooden clothespin (without metal spring), a party toothpick with plastic fringe, a piece of sponge, a metal jack (colored), a rubber garden hose washer, a penny, a plastic straw (cut to 3”), and a pencil-top eraser

What to do:
1. Discuss with students different ways of grouping objects. Objects can be grouped together based on similar features such as color, size or shape. Ask why it is important to group objects.
(Example: We are grouping when we organize the utensil drawer in the kitchen to separate knives, forks and spoons.)

2. Introduce dichotomous keys as one way of grouping and identifying. Introduce the term “dichotomy” and show how this is important in this kind of classification scheme.
(Example: The people in this room could first be grouped into categories of eye color. A couplet for this might be: “People who have brown eyes” and “People who don’t have brown eyes.”)

3. Divide the students into teams of two or three and give each team a Bag of Odds and Ends and the worksheet Key to Odds and Ends.

4. Students should choose one item from the bag and follow the key in order to “identify” the object. After making the appropriate choice in each couplet, the team will then follow the directions on the right hand side of the key. The directions will either indicate that the team must continue onto another set of questions, or it will present the team with the correct identification of the item. In this case, identification involves assigning the object a particular letter. Students can write the object name next to the appropriate letter on their worksheet, or copy this information into a notebook.

TEACHING TIP:
Students often “switch” their items as they proceed through the key, choosing an item that matches the positive statement. That is, a student classifying a paperclip, after the statement “Object not made of metal”, might drop the paperclip and pick up a toothpick, since it makes that statement true. Have students select one item and close the bag, only reopening and selecting a new item once they have identified the original.