Say there's a girl and a boy, and they're madly in love with each other, but they live on opposite sides of a lake that neither of them can cross. The boy wants to marry the girl and buys a ring to propose. There's a ferryman who goes back and forth on the lake. In his boat he carries a box on which you can put any lock. If you put something in the box, he'll take it across to the other side, but unless the box is locked he'll steal whatever you give him before he reaches the shore. Either way, he can go back and forth across the lake as often as you ask him to. The boy has one lock and the key to his lock, and the girl has another lock and the key to her lock. How does the boy get her the ring?
-- Catherine Chung, The Tenth Muse, Chapter 9