You want to send a valuable package to your friend.
You cannot go yourself to deliver the package and you can send it through a courier boy.
Since the package is valuable you cannot trust it with anyone.
The package has a locking ring which is large enough to have a locks attached to it and you have several locks with keys.
However, your friend does not have the key to any lock that you have.
You cannot send the key in an unlocked box since it may be stolen or copied.
How do you send the valuable object, locked, to your friend – so it may be opened by your friend?
If you get the correct answer, share it with your friends on WhatsApp, Facebook and other social networking sites.
Send the box with a lock attached and locked through the courier boy.
Your friend attaches his or her own lock and sends the box back to you.
You remove your lock and send it back to your friend.
Your friend may then remove the lock she or he put on and open the box.