When you read the title of this post, did you think Sesame Street? Foo Fighters? Or, like me, both?

Recently, Geoff shared seven (sneaky) activities to get students talking mathematically. One activity, ‘odd one out’, involves having students pick the one mathematical thing that doesn’t belong. This reminds me of one strategy used by Dr. Marian Small to create open questions – asking for similarities and differences.

Here’s my ‘odd one out’ question:

**Which of the following quadratic functions doesn’t belong?** (Dr. Small might ask “Which of these four functions are most alike?”)

Students might say,

because it does **not** cross the *x*-axis

because it is a vertical **compression** of *y* = *x*²

because it is a horizontal translation to the **left
** because it opens

**down**

I carefully chose the values of *a*, *p*, and *q* in *y* = *a*(*x* – *p*)² + *q* so that students could reasonably argue that any one of the functions could be picked as the odd one out. Because I am not looking for one particular answer, each student should be able to confidently answer the question and contribute to a mathematical discussion. Planning disagreement is key; it means students will have to justify their mathematical thinking.

Sneaky.