Which One Doesn't Belong?

The Triangle Man, Christopher Danielson, is up to something again. As a service to all parents around the world, he has created a deck of slides and has asked a very simple question: which one doesn't belong? There are no right answers, just a good discussion that is supposed to ensue. It's free to download, although I really hope he can make a ton of money off of it. I strongly encourage any parent to download it and try this out.  I have a 4 year-old, a bit too young to have a complex discussion, but why not try it out?  Sooooo, here's the unabridged version:

In the first slide without any coaching or prompting and then on every slide thereafter, I asked him "Which one doesn't belong?" That's it. His answers were pretty interesting, ranging from a color (or absence of a filled color) to the pointy shapes to the shapes that have something cut out. I love watching my son think and hearing him explain.  In light of an article that Matt Vaudrey posted recently on Twitter, I'm also trying not to celebrate every time he answers, but just acknowledge that he gave a response.  One cool observation is that he never asked me if he was right or asked what the right answer was. He's a kid, untainted by the repetition of right/wrong in education so far.

I plan to do these with him every couple weeks to see if his answers change and, as he gets older, ask him to justify his reasoning a bit more with mathematics.

For now, it's just shapes. And it's fun.

Happy "Outlier Shape" Fishing