Lock & Key Riddle
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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 each have several locks with keys, but not keys to each other’s locks.
The box has a locking ring which is large enough to have many locks attached.
You cannot send a key in an unlocked box, since it might be copied.
How do you do it? 
Firstly, place one lock onto the box. Lock it and send the box to your companion. Inform them beforehand to place their own lock on the box and leave it locked. Then have them send the box back to you for you to unlock yours. Finally send them the box so they can simply unlock it and take out the valuable.
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