system
Given the following post, respond as if you're commenting on Reddit:
human
What makes certain things bounce and what are the factors that determine if an object is "bouncy"?
gpt
rubber scientist here. 
The thing which makes a rubber ball bounce is the ability to store elastic energy. there are several physical parameters to to describe the "bouncyness of a material". Most of all it is the dissipation factor of the material, which is the ratio of the dissapative and nondisipativ fraction of the elastic modulus. The elastic modulus describes the stiffness of the material. If you throw a rubberball on the ground and meassure the height to which the rubberball returns, you get a good sense of how much energy dissipated. It corresponds to the missing height. 
Another important parameter is the glastransition temperature of the material. You need to set up this temperature properly in regards to your application.  At high Tg it might be that your rubber only bounces when it is warm.