They look the same. They feel the same. What’s the difference? You’ll have to drop them to find out.
You say you don’t have a pair of your own? Alright, you can check out the video. Warning, though – it might make you want a set. They are immensely fun to play with.
Choose your format:
You can see that one bounces and one stays on the ground. Why? They are made of two different materials: Polychloropene (trade name Neoprene) and Polynorbornene (trade name Norsorex). Both feel nice and rubbery and are about the same density. Both are pretty elastic – when you squish them, they return to shape nicely. Polychloropene is very bouncy – it wants to snap back to shape quickly. Polynorbornene is an impact-absorbing material that can be used for things like shoe insoles and high-tech golf balls.
Okay, with help from the data sheet (pdf) that comes with these emotional little guys, here’s the technical explanation: the two materials differ in hysteresis, which is a measure of the delay in the return to original state after a change is introduced. The happy ball (polychloropene) has low hysteresis, meaning it wants to get back to normal now! so it bounces right back into shape and into the air. As it bounces almost as high as you dropped it from, you could say that it has a high coefficient of restitution. The sad ball (polynorbornene) has high hysteresis, so when you deform it by dropping it against the floor, it only slowly returns to shape. Its kinetic energy is dissipated as heat. There are some other suggested experiments for these, but what could possibly be cooler than to weird people out with your non-bouncy rubber ball?
You can get happy and sad balls (also creatively called happy and unhappy balls) from many science education suppliers, but Arbor Scientific seems to have the best deal at $3.50 a pair.