==> logic/locks.and.boxes.p <==
You want to send a valuable object to a friend.  You have a box which
is more than large enough to contain the object.  You have several
locks with keys.  The box has a locking ring which is more than large enough
to have a lock attached.  But your friend does not have the key to any
lock that you have.  How do you do it?  Note that you cannot send a key
in an unlocked box, since it might be copied.