Question: Tom wants to send Sally an expensive ring through the mail because they live thousands of miles apart. The only problem is that everything sent through the mail is stolen unless there is a lock put on it. Both Tom and Sally have plenty of locks and keys but they don't have any of the same locks or keys.
How can Tom send Sally the ring without it getting stolen?
Answer: Tom puts one of his locks on the box with the ring in it. When Sally gets it she puts one of her own locks on the box and sends it back to Tom. Finally Tom takes his lock off of the box and sends it back to Sally who can then unlock her own lock and retrieve the ring.