As part of an upcoming “TNIFHS” post, I wanted to include one example of what Christopher Danielson’s approach to a better shapes book might look like in a high school math class. [Update: Christopher’s Which One Doesn’t Belong? is available from Stenhouse; from Pembroke for Canadians.] But then I had some fun with this and created a few more. In each set, a reason can be given for each of the four options being the odd one out. I’ve done this type of thing with numbers and equations before. Worthwhile, but not what I’m going for here. Each set below is pictorial. Also, I went naked; I stripped the graphs of grid lines and ordered pairs. More noticing properties, less determining equations. Aside from graphs, where else in secondary mathematics might this fit? The last two images below are my attempts at answering this question.