This is a fairly simple problem that can cause some people trouble. Kevin and Sophia want to mail high confidential letters between each other. They know that the mailman is crooked and opens the letters. Each of them has their own key and matching lock, and Sophia has a wooden box she plans on locking.
Neither key can be used on the other lock. Using the box, how can the two successfully mail letters between each other without the mailman seeing?
1) Sophia sends the locked box with her letter to Kevin. 2) Kevin adds his lock to the box and mails it back to Sophia. 3) Sophia gets the box, removes her lock and mails it to Kevin. 4) Voila! Kevin can now open the box using his key.