Brainteaser 1 - to warm up
Two friends collect stamps. If one of the two has two of the same stamps, he trades it with his friend for a different stamp.
The two live in Munich and Hamburg and therefore mail the stamps to each other on those occasions.
But at some point, more and more frequently, the envelopes arrive open with the stamps stolen. So the friends decide to mail the stamps in a metal box with a padlock. For that, they each buy a metal box and a padlock. How can the two of them mail the stamps in the metal box to each other safely without first having mailed each other the keys?