Learned something today!

I always thought the red flag went up when you had mail in your box.  I assumed there was a mechanism inside...or, less likely, that the mail man raised the flag to let you know.  Quite clearly I was wrong because the flag stayed resolutely down, even when we had mail.    However, my American friends informed me today that the mail box is a two way service.  If you have a letter to post (already stamped obviously) you put it in the box and raise the flag to tell the mail man "Collect my letter for posting before you put new mail in the box!"   What an excellent service!  I had no idea!