system
Given the following post, respond as if you're commenting on Reddit:
human
mass-energy equivalence or E=mc2...why the speed of light squared?
gpt
This is a frequently asked question. 

Here's one part of the answer: "c" is not really just the speed of light. "c" is the fastest speed anything can achieve in the universe. It's the universal speed limit. Light moves really fast. At the speed limit (in a vacuum). It isn't the only thing that moves that fast, but it's the most famous thing.

But for example changes in gravity also propagate at that speed. So you could also call "c" "the speed of changes of gravity."