# What is *Which One Doesn’t Belong?*

A Which* One Doesn’t Belong?* set consists of four objects with interesting similarities and differences, chosen so that any one of the four can be the one that doesn’t belong.

*Which One Doesn’t Belong?* is an invitation to conversation; a way to open our minds to new possibilities.

*Which One Doesn’t Belong?* is a mathematical routine that values divergent perspectives and encourages a focus on truth.

*Which One Doesn’t Belong?* is a prompt founded on the idea that children—all learners, really—are intelligent and insightful. They bring value to a community through their diverse ways of thinking and experiences.

In a *Which One Doesn’t Belong?* conversation, the measure of what’s *right* is what’s *true.*

# Where did *WODB *come from?

It all began with *Sesame Street*, of course. A typical *Sesame Street* set had three identical objects and one that was different—three large circles and a small one, for example.

Various folks messed around with that structure. For exampmle, the *Bridges *curriculum has examples of sets where—depending on your perspective—any one of the four objects is the one that doesn’t belong from at least as far back as the 1990’s. Sometime after 2010, Megan Franke used a WODB set in a professional development setting that Terry Wyberg attended. Terry did the same in one that Christopher Danielson attended. Christopher used the structure as a foundation for a shapes book for children and families. He shared his developing ideas about that on Twitter, which inspired Mary Bourassa to establish a website.

Tracy Zager and her team at Stenhouse had the vision to publish Christopher’s shapes book together with a teacher guide that addresses how children learn geometry, how teachers use the routine in their classrooms, and the rich mathematical ideas that learners of all ages express when given the opportunity.

In 2024, Mary is retiring from running that website, and the content has moved over here to Christopher’s Talking Math with Your Kids site, just in time for the tenth anniversary of the first draft of his picture book and teacher guide.

# Where can I learn more?

There is a teacher guide, available from Stenhouse (now a division of Routledge), and from Pembroke Publishers in Canada.

Lots of folks have used the routine in their classrooms and written about it. Here are some examples:

- Chris Hunter General WODB Post
- Steve Wyborney Imposter Sets
- Alex Overwijk Grade 10 Applied Math
- Mary Bourassa Calculus
- Tina Palmer Geometry
- Jennifer Wilson Geometry
- Beth Ferguson Graphs of Functions
- Nora Greene Number Sets
- Rachel Fruin Shapes
- Bryan Anderson 3D Shapes

A number of teachers have spent time thinking about the language of *belonging* as it’s used in this routine. Christopher Danielson wrote about that in a post titled “On Belonging, and Not“.

# OK, but seriously. Which one really doesn’t belong?

Upper left. The one in the upper left is always the right answer.