The idea is that students would have to logically determine what is present or absent in each of the objects shown, and find a way of completing the set. This can act as a stepping stone between students finding which one doesn't belong and creating their own set. Chris showed a great way of organizing characteristics that would look something like this for the above set:
Since seeing Chris do it this way, I have used this type of table to sort through submissions to the site. I love how logical it is and how it makes it obvious whether you have a solid set - one where each group of 3 objects have something in common that the 4th does not.
Here are two more that I made:
I hope you will join me in creating new complete and incomplete sets! Please use the submission form here and let us all know how you are using these with your students. I have an ever-growing list of other improvements/additions to the website. Please let me know what you would like to see by leaving a comment. Thanks for all the support!