| An expanding and contracting ball marketed as an educational toy and object of art. It is a mechanical structure made of slightly curved links (or struts) joined by pivots into "scissor-pairs." These basic units are connected by hubs into larger units. The opening and closing of the scissor-pairs results in the folding and unfolding, expanding and contracting motion of the sphere. Many other structures use similar linkage systems, such as folding gates and clothes-drying racks.
The Hoberman Sphere is technically not a sphere. It is actually a spherical polyhedron known as an icosadodecahedron. That means it contains 20 (icosa) triangles and 12 (dodeca) pentagons.
The sphere retains its shape because it is composed of six circles. Because it is a closed unit, the circles strengthen the structure. In addition, each intersection reinforces the structure. Since there are so many joints, the entire structure remains strong. Also, since the sphere retains its shape during the contraction and expansion process, its strength is maintained throughout the transition.
The largest Hoberman Sphere is located at the Liberty Science Center in Jersey City, NJ. It is made of aluminum and stainless steel. It is 18 feet in diameter when fully expanded. It expands and contracts automatically using motorized cables and a pulley system. I am told there exists an equally large sphere in Los Angeles.
The spheres you can buy, however, are made of plastic. Currently there are two sizes. The original size is 9.5" inches and expands to 30", a 30-fold increase in volume. There is also a mini-sphere, which is 5.5" and expands to 12", a 10-fold increase in volume. Update: Hoberman now markets "microspheres." They are 3" when expanded and are composed of only 4 circles and is thus weaker and does not retain its shape as well. Marketed as a finger-toy, it is definitely not as cool at the bigger ones.
The larger sphere is a lot more fun and usable. As someone avidly interested in skill toys, I view the sphere as one. There are several tricks you can perform with it involving tossing, spinning, and balancing. The basic concepts in tricks are:
spinning the sphere rapidly will cause it to expand due to angular acceleration (centripetal force)
balancing the sphere on a hub from underneath will cause it to contract
By expanding these basic concepts you can build tricks. My favorites include a simple spin toss to the ground that makes the sphere bounce back towards you, and the two-handed spin where the sphere is spun with fingers on the horizontal axis opposite each other, twirling the sphere rapidly. It's an amazing stress reliever. When you get tired, you can just stare at it as it contracts and expands.
Hoberman Spheres are also frequently seen at raves. E-tards like to trap themselves within it.
Sources: flyer that came with my Hoberman Sphere | 
