I actually had this conversation once:
Co-worker: I have a question.
Co-worker: *some inane question about something I wouldn’t even have remembered even if it happened five minutes ago, let alone a year and a half ago*
Me: *simple answer*
Co-worker: Well, that’s not in the instruction manual.
Me: But that’s the answer. Trust me.
Co-worker: But I don’t trust you, it’s not in the instruction manual.
Me: *deep breath* I WROTE the instruction manual. Trust my answer.
Co-worker: BUT IT’S NOT IN THE INSTRUCTION MANUAL. WE NEED TO FIND THE ANSWER.
Me: I’m TELLING you the answer.
Co-worker: BUT IT’S NOT IN THE MANUAL.
Me: I WROTE THE MANUAL!
Co-worker: Then why isn’t it in the manual?
Me: Because it’s a minor detail. I guess I assumed it would be taken as read.
Co-worker: Well, it’s not. It’s not in the manual.
Me: Why can’t I just tell you? We are the only two people who work in here.
Co-worker: *something, something, something, grumble, grumble MANUAL grumble something*
Co-worker: *something, something, something, grumble, grumble MANUAL grumble something… but in a higher voice*
Me: If I wrote this down on paper, would you listen to me then?
Me: *grabs a pen and a post-it and writes the answer down* Here.
Co-worker: *satisfied* Thank you.
UPDATE/CLARIFICATION (@11.36 pm): In case it wasn’t clear, I actually DID write the manual.