Harry and Sally met online and fell deeply in love. Harry wishes to send Sally an engagement ring via snail-mail but...

The local postal service pilfer anything that is not locked in a box with a padlock.

Harry and Sally both have lots of boxes (and lots of padlocks) but none where each has a copy of the key..

How can Harry get his Ring to Sally ?

/mh

PS.. i _dont_ claim credit for coming up with this puzzle (or the solution)

PPs: For Bonus points, whats this solution better known as?