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