An expandable breathing ball, also known as a Hoberman sphere, is a colorful, geometric toy that can expand and contract. It's made of interconnected plastic joints that allow it to grow from a compact, closed shape into a large, open sphere and then collapse back again. This movement mimics the action of taking deep breaths, expanding as you inhale and contracting as you exhale, which is why it's often used as a tool for teaching deep breathing exercises and mindfulness techniques. The visual and tactile nature of the breathing ball makes it a useful aid for both children and adults to practice relaxation and stress relief.