How do you make it so Facebook doesn't say that you read someone's message?

[Trevor]

So you know how if you selected someone's message they sent you on facebook, and it tells the other person that they saw it, how do you make it so that doesn't happen?

[Mikill Øxveifr]

There used to be a Chrome extension called FBUnseen, but it recently said it had to be disabled because it violates Facebook rules.

[abraXus]

you just mark it as unread

it's explained here

http://abrax.us/Facebook/DeletingMessages.php