I like this analogy of the public key very much. The analogy is that of a locked store front door with a mail slot. The mail slot is exposed and accessible to the public; its location (the street address) is in essence the public key. Anyone knowing the street address can go to the door and drop a written message through the slot. However, only the person who possesses the matching private key, the store owner in this case, can open the door and read the message.