system
Given the following post, respond as if you're commenting on Reddit:
human
What are we smelling when we open a fresh can of tennis balls?
gpt
Plasticizer! Some plastics and rubbers have chemicals called plasticizers that enhance the material's flexibility. One of the characteristics of plasticizers is that they're volatile, meaning they naturally want to evaporate. The smell you get from a fresh can of tennis balls is the evaporated plasticizer that has built up in the canister.

Plasticizer evaporation is also the reason that extremely old tennis balls become brittle.