system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why/how do erasers get stale?

I’ve noticed that erasers on the ends of pencils and sometimes standalone erasers will harden so they just smudge pencil, or they’ll break on use. Aren’t they just rubber? Why does this happen?
gpt
The erasers on the ends of modern pencils are usually some form of synthetic compound, possibly with some rubber, possibly without.  The most common ingredient is vinyl, a petroleum based compound.  Petroleum compounds are great for making flexible objects that perform elastically (like erasers and rubber bands), but those properties are partially due to the volatile chemicals in them.  Exposure to light and oxygen accelerates the evaporation of these volatiles from the compounds, thus making the eraser go “stale”.  The compounds that held everything together in that flexible matrix are gone, leaving only the solids behind, and a lot of times they are barely held together until you start to use them, at which point they crumble.