When does Facebook show that someone has "seen" your message?

[Angi]

I was just wondering at what point does Facebook show that someone has seen your message. Is when they actually click on the message or when they click on their inbox and see that the message is there? Hopefully this makes sense lol. Thanks in advance

[abraXus]

it depends on if you are using a computer, or a phone, if you clicked on the reply box or closed the chat window, if the person read it and then marked it as unread - there are too many variables to really give you an accurate answer

[M]

If the person clicked into the actual message, then it shows as read. Or if they move their cursor into the little chat box once a new message pops up.

It doesn't say "seen" if they just click the red notification that indicates a message and stop after that step.