system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is happening at the atomic level when a rubber band becomes hard and brittle?
gpt
Not so much at the atomic level, because rubber is not homogeneous, but at the molecular level there are a few things happening. 

Ruber is an "elastomer", meaning that there are lots of long chains of molecules that are bunched up in their natural state, but can be pulled out into long strings when you apply some force to it. 

There are oils in rubber that keep these elastomers from contacting one another and making bonds between them , which would keep them from unwinding. Over time, these oils evaporate, allowing the new bonds to form, and UV light destroys the elastomers, and eventually the rubber solidifies, then disintegrates.