In this strategy, teachers give students the opportunity to determine which items belong or do not belong in a group. Teachers present students with four figures, diagrams, graphs, or expressions and ask the students to determine which one doesn't belong. Teachers should strategically pick figures, diagrams, graphs, or expressions that could each not belong for different reasons. The objective of this activity is to support students to justify their thinking using mathematical terms precisely.
If you are using this as an Open Up Instructional Routine in a lesson, locate this Instructional Routine during specific warm up activities within the lesson.
If you are not using the Open Up Curriculum, create four figures, diagrams, graphs, or expressions that align with the current content you are teaching. Consider using the "Which One Doesn't Belong?" resource below.
If you are creating your own group of four figures, diagrams, graphs, or expressions, be sure to pick figures, diagrams, graphs, or expressions that could each not belong to the group due to different reasons. This will support students to justify their thinking.
You do not need to only have four items. The number of figures can be changed based on your individual situation and lesson goals.
Model for students by showing them how you would determine which item might not belong in a group of four.
Show the students four figures, diagrams, graphs, or expressions and ask the students to respond verbally or in writing to the question, "Which one doesn't belong?"
Be sure to ask students to justify their answers by responding to questions like, "Why?" Or, "How do you know that?"
Remind students to be precise in their explanations.
Use this activity as a warm up to support students to reason mathematically and justify their thinking, or as a lesson closer in order to assess students’ comprehension of new material.
Provide English Learners with sentence starters to support them to formulate their responses. Consult the "Accountable and Academic Student Talk Stems" strategy in the BetterLesson Lab for resources or for more information about how to use talk stems with students.
Explore the "Learning to Sort -- Same and Different" lesson by Kindergarten Master Teacher, Christina Yingling-Smith, to see how an early elementary teacher introduces the strategy of "Which One Doesn’t Belong?" to her students.