When does a facebook message become 'seen'?

[Kevin]

More specifically, does the message get marked as 'seen by' you when you see it in your notifications, and read it in your notifications, without actually opening the message box, i.e. not clicking on the notification?

[Daniel]

It Becomes Seen When you Open the Message Up

[Jeanne]

When the person click on the message, it shows "seen by"

[Drizzy]

It only happens when it's marked as read, and opened your message