Secret Mail Puzzle
You want to send a secret message to your friend Alex in the mail.
But Sam, who you don't trust, has access to all your mail.
So you put your message in a box with a lock.
But you are not allowed to send a key!
How can you get your message through securely?
Solution to the puzzle:
Send the box with the lock to Alex.
Alex can't open it, but can put another lock on the box.
Alex sends this box with the 2 locks back to you, you
unlock your lock and send it back to Alex again.
So there is just Alex's lock on the box and Alex can now open it.
