It’s a Dichotomous World
Standards Addressed
6.1.2
Give examples of different ways scientists investigate natural
phenomena and identify processes all scientists use, such as
collection of relevant evidence, the use of logical reasoning,
and the application of imagination in devising hypotheses and
explanations, in order to make sense of the evidence.
6.2.5
Organize information in simple tables and graphs and identify
relationships they reveal. Use tables and graphs as examples
of evidence for explanations when writing essays or writing
about lab work, fieldwork, etc.
Objectives
The student will be able to…
 Organize items based on their various properties
 Create a key that their peers can use to determine the name of
various items
Required Materials

For each Group
o Nail
o Penny
o String
o Button
o Screw
o Sponge
o Paperclip
o Wire


Extension 1
o Metric Balance
o Metric Ruler
Extension 2
o Bolt
o Plastic wall anchor
o Tongue depressor
**These items were chosen based upon easily identifiable properties
and their relative low cost. The sponge can be sections cut from a
larger sponge.
**Materials for Extension 1 should only be used if they are already
owned.
Lesson Introduction
Organizing things by grouping those with common characteristics is
called Classification. Everyday, you classify items by sorting objects
and putting them in to a specific place; sock drawers, computer files,
items in the garage.
It is also an important part of the scientific process to classify things
because it highlights the differences and similarities between different
things. Many concepts in science are based on facts produced from
classifications; an example of this is in chemistry. The periodic table
orders atoms based upon their physical properties. Geologists classify
minerals by observable physical and chemical characteristics. In
Biology living things are classified by a many different taxonomic
categories.
Scientists use a variety of classification
methods to organize the world around
them and a common way to arrange the
classification system is to use a Key. A
Key is usually organized into a series of
choices that lead to the name of a given
item. Keys are able to guide others to
use a system of classification to classify
items. One of the most commonly used
keys is the Dichotomous Key.
Dichotomous means divided into two
Figure 1: Basic Dichotomous
Key for four animals
parts, therefore, in Dichotomous Keys
two choices are always given in each step to narrow down the
classification for an item until only once choice is left (Figure 1). In this
activity the students will create their own dichotomous key for some
common items. This Classification activity can be used as a lead-in
activity for many of the other PIE lessons.
Pre-Activity
To introduce the concept of a Dichotomous Key to students have each
student (or half of the students depending upon class size) take off
their right shoes and place on the floor in the center of the classroom.
As a class develop a dichotomous key or system of dual divisions with
the help of the instructor. The classification system should be drawn
on the board as the divisions are made similar to figure 1. Another
example is in the Chart Examples section after the references.
First, students should observe the shoes to note similarities and
differences. Then they can choose a category that some of the shoes
have and some do not. Have them write it down. For each of those
groups students can subdivide the shoes again into a category that
describes some but not others. Every time they divide a set of shoes,
it should be into 2 groups. These divisions are based on some visible
trait that some have and others do not. They should continue dividing
the shoes until there is only one in each category. This is a
dichotomous key. They can test their key by tracing some of the left
shoes through each of the descriptive categories to see if following the
divisions leads them to match it with its mate.
Activity
Break students in to groups of 3-6 and give each group the bag of
materials from the materials list. Tell the students to create a
dichotomous key for the items you have given them. Many will notice
a variety of properties and will have many different ways of separating
the items. Allow the students about 10 minutes to separate the items
based upon the classification system they develop and write out their
dichotomous key.
After the students have had 10 to examine the items, hold a class
discussion on the properties that they have observed. Tell them that
we need to establish some terminology so that groups can compare
the keys that they have create. Have each group give a property that
they used and write it on the board; allow other groups to comment on
the similar term they used. This allows the similar terms to be
grouped and defined for the Assessment portion of the lesson. The
following questions can be used in the class discussion or can be given
to the students for a journaling assignment.
Questions
1. Why did you classify the objects the way you did?
2. Compare your systems with those of other students. Did you all
classify the objects the same way?
3. Do you think scientists always classify things the same way or
agree on systems of classification?
**Answers to these questions can be found after the references.
Assessment
The best assessment for dichotomous key is for another group in the
class to use it to classify their items. Collect a key from each group
and distribute the key to a different group. Have the groups evaluate
the key on how well it works for identifying each item.
Extension
1. Set out a metric balance and a metric ruler. These items should be
available for student use in classifying the items. It also allows the
student to practice using these tools. The mass and length of items
could potentially be used to classify all of the materials, if some
groups wish.
2. After the students have made their dichotomous keys, give them
the materials from the Extension 2 materials list. Have the
students name the items based upon their key. Then have the
students make additions to their key to add in the new items.
Reference
http://www.cns.uni.edu/LakeStudy/Education%20Pages/new_page_4.
htm
Chart Examples
Shoe Dichotomous Key
Shoe Laces
YES
High Top
YES
Air Jordan’s
NO
Sneaker
NO
Slip on
YES
Sandal
NO
Velcro Sneaker
Answers to the Questions
1. Why did you classify the objects the way you did?
These answers will vary. Certain physical properties will most likely
be listed to defend their answer.
2. Compare your systems with those of other students. Did you
all classify the objects the same way?
Hopefully the answer is NO. It is possible that groups order their
classification the same, but the chances are very unlikely that more
than two groups are the same.
3. Do you think scientists always classify things the same way
or agree on systems of classification?
This is also a NO. There are many classification schemes used, but
some are more accepted than others and are used to ensure
continuity in the scientific fields.