The Spring Ball is based on the principles of springs (which are objects that exert force directly proportional to the distance they are deformed), and on the principle of conservation of momentum. The momentum refers to the balls, which are bouncy rubber that undergo highly-inelastic collisions. Two bouncy balls are attached by a spring. This can create some very interesting physical interaction, as the spring can pull the balls in strange, transient patterns as various forces are applied to the balls/spring (from the child, from gravity, from the impulse of the balls colliding into things, from the spring as the balls pull it away from its default length).
The spring ball is a toy intended to foster creativity, and as such, there is no set-in-stone way to play with it, but there are numerous fun games that can be invented for it, limited only by the child's imagination. A few simple examples are: holding one ball and bouncing the other off of it (can be modified by holding the center of the spring and bouncing the balls off of each other in a "butterfly" fashion); trying to throw the spring ball in certain ways to produce a desired effect, such as, just swinging/throwing it randomly against the floor/wall and watching the results, trying to get a good spin by the balls about the center of the spring, trying to throw it such that one of the balls stays off of the floor for as long as possible (there are various different tricky throwing methods that might work for this); or it could be used to add an extra layer of craziness to raquetball/squash/tennis [supplies not included].
An added feature with the spring ball is that there are three interchangeable springs of varying stiffness. This further expands the number of different ways that the spring ball could be used in.
Production is surprisingly simple. The toy is two "super-balls" attached by a spring. The loose spring will have a tension comparable to that of a slinky. The other two springs will be roughly 2 times and 4 times stiffer, respectively (still awaiting product testing results to determine ideal spring constants). The only somewhat tricky part of construction is attaching the spring securely to the ball, and also making it removable.
Almost all off the force on the spring will be laterally along the axis of the spring, so it the springs are screwed in securely, given that there is little rotational force applied, the springs should be secure. Thus, all that is needed is a spiraling hole into the ball slightly larger than the spring thread-size (or possibly the same size, when one considers that rubber can deform significantly). The hole should be tight enough such that it takes a somewhat significant amount of force to insert or remove a spring.
The toy is decently safe. It is slightly less safe than a springless-bouncy ball, but not significantly. The toy is also very durable. Potencial breaks are: springs wearing out, and the spring ripping through a ball. Replacement springs and balls could be packed with it, it would depend on just how long the balls and springs are capable of lasting. Estimated price: could be as low as $10, all ages.
Other toys: Whirlygig, Jump Roller.