system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Ouiji Boards

How the hell do they work?
gpt
They don't.  The planchette moves either because one of the participants chooses to make it move in order to impress the others, or (more often) because tiny subconscious movements on the part of the users cause it to seem as though it is moving on its own.  This is called the ideomotor effect, and is also how water divining (dowsing) "works".

One interesting thing about ouija boards is that if none of the participants can see the letters, they suddenly stop producing messages.  This is a pretty serious indication that nothing paranormal is happening here :)