For mathematics instructors and tutors, math educator Mary Bourassa has created this collection of "thought-provoking puzzles for math teachers and students alike." Each of these puzzles presents learners with four items and asks students to identify and explain which item is the odd one out. However, these puzzles feature one notable twist on the "Which One Doesn't Belong" puzzle: every single item in each grouping could be considered the one that does not belong depending on what criteria learners chose to examine. As a result, these puzzles offer an opportunity for learners to engage in open inquiry and conversation about mathematical concepts. This collection features three kinds of groupings: shapes, numbers, and graphs & equations. As of this write-up, this collection includes over 150 puzzles. Visitors are invited to submit their own puzzles to add to this collection.