Lock on a box
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.