Which one doesn't belong?

Website created by Mary Bourassa, high school math teacher from Ottawa (Canada) dedicated to providing thought-provoking puzzles for math teachers and students alike. 

There are no answers provided as there are many different, correct ways of choosing which one doesn't belong. It is open to new submissions by teachers from all over the world.

(Source: http://wodb.ca/about.html)

Also an interesting resource for class discussions.

You can find it on: http://wodb.ca/index.html