If you clear your chat window on facebook does it clear for another persons?

I was wondering if you send someone a message like 'hey, how are you' and they don't read it because they change from online to offline when you send it or because this chat is ultimately crap, if you clear chat box using that option at the top will it clear it for the other person so you can send them... show more I was wondering if you send someone a message like 'hey, how are you' and they don't read it because they change from online to offline when you send it or because this chat is ultimately crap, if you clear chat box using that option at the top will it clear it for the other person so you can send them that again without looking desperate to talk to them or is there any way of doing this at all?
Thanks
Update: Also how long does a message you sent them appear in their chat box for you before it disappears?
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