Puzzles/How do you ... ?/Sending a Package

Dmitri, who lives in Moscow, has bought a pretty necklace for his fiancee Ana, who lives in St Petersburg. He wishes to send the necklace to her via post, but due to problems in the Russian postal service, he fears the package will be opened and the necklace stolen. There is a considerable distance between the two cities. Both Dmitri and Ana own a number of locks, chains, and keys. Ana does not, however, own a key to any of Dmitri's locks, and vice versa. How can he send the necklace to her? Solution