During world war two English spies named Rob and Ben were involved in passing highly secretive maps, documents etc., from Germany to England. But both of them didn't know each other and were strictly restricted from meeting each other for security reasons.
Rob will collect the documents and lock it up in a mail box with a strong lock on a main road of Germany. Ben would come there later, open the mail box and take away the documents and then lock it up. Both had their keys to the same lock.
Once Ben lost his key to the lock. So he could not open it.
Rob understood it and thought of changing a new lock. But he could not pass the key to Ben. He could not also keep the box open as somebody may look into the box and they might be caught. Both must reach that mail box only on different times of a day. If Rob comes in the morning, Ben should come in the evening. Also, both must not meet each other not even through a messenger. Actually Rob didn't even know who Ben is.
They were intelligent spies and found a means to pass on the documents through the same mail box. How?