Which one doesn’t belong, the red square, the blue square, or the blue circle?
Surprising to most people, the answer is the blue square. Here’s why.
- The red square is the only red object. The others are blue.
- The blue circle is the only circle. The others are squares.
- The blue square is not the only anything. It shares a trait with each other object.
Since the red square is unique by color and the blue circle is unique by shape, the blue square is the only object that is not unique, which makes it the most logical choice for the one that doesn’t belong.
Visually:
We recognize that questions like ‘Which doesn’t belong?’ can feel ambiguous or even unfair, so we tested alternatives — because when wording misleads, even smart humans and AIs can miss that the puzzle is about reasoning, not perception.
A variation of this puzzle first appeared in Martin Gardner’s Mathematical Games column in Scientific American in 1972.