Which One Doesn't Belong?

One thing we do, to help us get good at sharing our reasoning clearly, is Which One Doesn't Belong?

We can choose any of the four pictures to be the one that doesn't belong, and we give the reason why we think it doesn't.

Here's an example:

Which one do you think is different, and why? You could write your answer as a comment.

Here's some of the things that the two classes thought:

Sometimes we get a bit of a discussion going.
This time, KM said the bottom right one was a diamond rather than a square.
Some children thought is was still a square, but slanting or turned round.