For this activity, you will need one small paper bag per student and the following types of candy (1-2 per student):
Place 1-2 pieces of candy in each paper bag and put each bag on a student's desk. Have it waiting on the students' desks when they arrive, but don't let them open or look inside the bag. This will spark their natural curiosity. When they open the bag and realize its contents are candy, engagement will not be a problem! (Although distraction - due to excitement - might! To keep this to a minimum, I clearly state to the class that they will only be able to eat the candy at the end of everyone completes the activity and stays on task. otherwise, I will collect it and give it to another class to enjoy. Knowing they will eventually get the candy will help to satiate them long enough to use it as a learning tool. I have provided other management options for the candy in later portions of the lesson.)
Next, I hold up pictures and names of several turtles that I have printed off of the Animal-World website. I ask the students to select the picture of the turtle that they just learned about in the video. I accept many guesses, asking students to justify why they believe this is the correct answer. They will come up with several ideas, but none of them will be correct. After a minute or two of students trying to select the correct turtle, I explain that none of them are the same species of turtle we saw in the video. I tell them that it is very difficult to identify a specific turtle species by just looking at them once or twice, and explain this is why scientists had to come up with a more methodical way of identifying animals correctly. This is when I introduce the idea of a dichotomous key.
I explain to my students that a dichotomous key is very useful because if you know about a animal group, you can know about an unfamiliar animal that is a member of that group. Conversely, if you know a member or two of an animal group, you can make some good guesses about the group as a whole.
Before moving on to the exploration part of the lesson, I want to help my students understand the term, "dichotomous key" and to understand why it is used to describe this tool. I ask the students to think of the word, "key" and to brainstorm several meanings. I narrow down their focus by having them discuss what "key" means in reference to codes or maps. I have students share out their thinking and draw or project an example of a map key for all to see. I ask them how this may be helpful to someone reading the map.
Next, I project the definition of "dichotomous" from Webster's online dictionary, and have students read it and discuss what this might have to do with the "key" we just discussed. I help guide them to the conclusion that a dichotomous key is a key for the identification of organisms based on a series of two choices or alternatives. This may be difficult to understand at first, but it becomes very clear as we progress, so it may be helpful to revisit this part of the lesson after students have had some practice.
Before moving into content related to the use of a dichotomous key, I provide a short activity that allows students to practice the use of a this tool using a familiar material - candy! I start by telling the students that I will successfully perform a magic experiment right in front of their eyes, but first I need a "lovely assistant". I select a volunteer and have them join me in the front of the room. I offer them a paper bag filled with different types of candy and ask them to select a piece from the bag, show it to the class, but not to show it to me. Once they have selected their candy, I explain that I will now tell them what kind of candy they have selected without ever looking at it.I even make them a deal - if I guess correctly, I get the piece of candy. If I am wrong, they get the whole bag! (This drums up excitement among everyone!)
I project the Candy Key Worksheet under the doc cam, and model use of the key to correctly guess the type of candy my assistant has selected. The kids are usually disappointed that I guessed correctly and some will even accuse me of cheating, but that quickly dissipates once I tell them that I will give them all a chance to perform the same experiment and earn some candy of their own.
I divide students into pairs and hand each student a copy of the Candy Key Worksheet. I have them open their bags and select a piece of candy without showing their partner. Then, they each get to guess each others candy by using the dichotomous key provided. If you provide two or more pieces of candy to each student, then they can perform multiple trials. However, one is enough to provide practice with the key.
As a class, we discuss how using the dichotomous key helped them figure out the classification of the candy they selected. I briefly explain how scientists use the same technique to categorize all sorts of organisms, and to share their knowledge about these organisms with other scientists.
I also provide context - a few examples of when scientists use a dichotomous key, such as to identify fossils, to find a specific endangered species, or to locate plants to treat a certain illness. It is also valuable to mention that online dichotomous keys have become more common, and are much more efficient, than their written predecessors, and show a few, if time permits. Some examples include:
For this portion of the lesson, I have provided several options. They are listed in order of scaffolding and support (most to least) for both the students and the teacher. Each option meets the same objectives and has students take a hands-on approach to using a dichotomous key to identify species within an animal group.
Option 1:AZ Fish Dichotomous Key (provided by the AZ Game & Fish Department)
This activity is very comprehensive, including 3 parts that increase in complexity.
Option 2:AZ Turtle ID Chart (also provided by the AZ Game & Fish Department)
This chart provides a graphic dichotomous key for identifying native species of turtles. I like this one, because we have three class turtles and the students enjoy finding which species we have in our classroom. I follow the same steps that are included in the above activity, but without the scaffolding and structure of the worksheet.
Option 3: Make your Own
Check online or contact your local Game & Fish department for dichotomous keys that depict animals native to your own region. They are easy to find and provide a relevant learning experience. If you can, take a nature walk and find an animal to investigate with a key or to create a key for. Students love this!
I use the student work in the Elaborate section to determine whether students have completely understood the material. As an extension or a final demonstration of understanding, you can also provide the following written assessment opportunities: