At the beginning of the 19th century, the Russian postal service was very corrupt. The mailmen opened and stole all the objects of the correspondence that was not protected. The only thing they could not open were the protected boxes with a lock.
Boris lives in Moscow and wants to send his girlfriend Natasha, who lives in St. Petersburg, a necklace as a birthday gift.
If you send the gift in a box without a lock, you will be robbed. If you put the necklace in a locked box and send the key in an envelope, the box will arrive, but not the key.
How can Boris, using only the Russian postal service, send the necklace to Natasha?
No need to send any keys!
Boris sends Natasha the necklace in a locked box. When he receives it, Natasha sends Boris the box again by adding a padlock of his. Boris receives a box with two locks, removes his and sends it again ... So he can finally see his necklace!