How does Facebook know when I saw a messege?

[Frankie]

If I sent my friend (who is offline) a messege, then they log on and press the messege icon, is that when Facebook determines when someone seen your messege or does Facebook determine it when they click on the messege itself?

Really important, Thanks!

[Navi]

When they open up the message, and it is visible on the screen, facebook says they have seen the message. However, the person who "saw" it may not have been looking at that part of the computer screen, so they may not have actually seen it. Understand?