Which One Doesn’t Belong – Three Weeks In

I started using Which One Doesn’t Belong this year as part of my normal rotation of warmups. I haven’t been very consistent because of some issues with the projector in my room, but I’ve been doing it about once a week in each of my classes. There are a ton of great prompts on the site, and I’ve found them especially useful as formative assessment for what features of graphs and equations students are fluent in and able to distinguish. It’s pretty telling when my Algebra-II students catch the negative slope right away, but don’t identify the y-intercept as quickly.




While most of the prompts I’ve used are review, especially for my Pre-Calc classes, it’s a useful and practical way to get students discussing and creating arguments, as well as quick hits on concepts and vocabulary.

All that said, I have some work to do making this a productive warmup for all of my students. I introduced these warmups with the prompt, “which one doesn’t belong?” As we discussed in each class, students made arguments for different graphs or equations, and I thought it became clear that there was a reason why each of the four didn’t belong. But I didn’t make that explicit, and now, three weeks into the year, a small but significant number of students take this warmup as an opportunity to name one easy graph or equation that doesn’t belong — and stop thinking. Many kids are into it, and there have been some great discussions, but I think not being explicit about the goal of the activity has created an opportunity for some students to opt out. I’m working to correct this, but if I had the chance to do the whole thing again, I would work to create a clear expectation from the first day that the goal, and the mathematical thinking students should be doing, is to find a reason why each of the four doesn’t belong.

Next up, incomplete sets!