which one doesn't belong

"Which one doesn't belong?" is an activity or a question format used for comparing groups of four items where participants are asked to identify the one item that does not fit with the others. The key aspect of this activity is that there can be multiple correct answers depending on the reasoning given — it is not about finding a single "right" answer but about justifying why one of the items stands apart from the rest based on its features or properties. This approach encourages discussion, reasoning, and deeper thinking, especially popular in math education to help students engage in math talk and argumentation. It is inspired by the Sesame Street segment "One of these things is not like the others," and has been developed into various formats including a book by Christopher Danielson and classroom routines. The activity values many perspectives since each item in the set may have a reason to be considered as "not belonging" depending on the chosen attribute or criteria. It is often used to foster inclusion, communication skills, and critical thinking without the pressure of a single correct answer.