system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between "good" erasers and "bad" erasers (one's that smudge the lead and don't entirely erase), and what causes it?
gpt
When the rubber dries out, it no longer rubs off from abrasion on paper, and is too hard to adhere well to pencil graphite.  So it's like smudging the graphite with a finger instead of a spunge.

This happens due to the passage of time and dry or hot environmental conditions.