system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly does Citizen's Arrest work?
gpt
Back in the day in England, sheriffs wanted to encourage people to help them catch lawbreakers so they allowed normal citizens to catch crime suspects.  This tradition exists now in the US in common law, but the specifics of it vary wildly from state to state.

Generally, to make a citizen's arrest the crime must be a felony and you must have witnessed it.  Some states allow you to arrest for misdemeanors or breaches of the peace or if you have probable cause to believe the crime occurred but did not witness the actual commission of the crime.

The most important point about citizens' arrests is that you do not get any legal protection from being sued.  Police officers get tons of legal protection--they really have to mess up bad to expose the State to liability, and ever worse to expose themselves to personal liability--but you will face strict liability for anything you mess up.

Finally, there are special rules for shopkeepers who believe that you have shoplifted something from their stores that allow them to detain you until the cops arrive.  That is separate from the citizens arrest powers we're talking about here.