system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is throwing a glass of water on someone considered Assault?

the other night at a bar, a fight broke out...someone threw a glass of water on the two fighting in an attempt to break it up and was arrested for assault.  Felt this was unfair, as water is harmless and they were attempting to break the fight up.    why is water considered a weapon?
gpt
"Assault" means different things in different states, so it can confusing.  In this instance, it is almost certainly being used to describe a crime that is sometimes called "simple battery" (or something similar).

This crime requires that (a) an offensive touch or contact is made upon the victim; and (b) intends or knows that their action will cause the offensive touching.

So, if a person causes water to physically touch someone, and knows that the person getting hit with the water would be offended by it, it's assault. 

From a nerdy legal point of view, the real question is whether this assault could be a justified use of force. If the person thought that by throwing the water they were using the minimum amount of reasonable force necessary to prevent a more serious battery, there might be a defense to the assault charges.