system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it so painful to rub salt on a open wound?
gpt
Salt is sodium chloride. Your nerve cells communicate partially using sodium. When a bunch of sodium rushes into a nerve cell, it "fires".

The salt that gets into the wound dissolves (splits into individual sodium and chloride), and because there's so much of it, the sodium rushes into nearby nerve cells, overloading them and causing them to fire. 

There are some nerve cells that are specialized to detect pain. Since the nerves that detect pain are constantly sending signals to the brain because they're overloaded with sodium, your brain interprets the constant signal as constant pain.