"Which One Doesn't Belong?" in math is an instructional routine where students are presented with four items—such as shapes, numbers, equations, or graphs—and must determine which one does not belong with the others. The purpose is to encourage mathematical reasoning and argumentation by identifying sets where three items share a common property and one differs. The key aspect is that multiple answers can often be valid depending on the reasoning used, as each item can potentially be the one that "doesn't belong" based on different mathematical characteristics like shape, symmetry, number properties, or patterns. For example, in a set of four numbers, one might not belong because it is odd while the others are even; another might not belong because it is prime while others are composite; yet another might be excluded because it breaks a certain numeric pattern. Similarly, for shapes, one might differ by color, number of sides, symmetry, or whether it contains curves or straight edges. This activity is used often as a warm-up or "tuning-in" task to reinforce concepts such as geometry, number properties, place value, or pattern recognition. It also supports different levels of student understanding by allowing multiple valid mathematical explanations for which item does not belong and why.

If desired, a more specific example or a set of items can be provided to illustrate how "Which One Doesn't Belong?" works in practice.