A wants to send a secret message to his friend B in the mail.
But C (A’s Friend), who A don’t trust, has access to all A’s mail. So A put his message in a box with a lock. But A is not allowed to send a key!
How can A send his message through securely?
Send the box with the lock to B.
B can’t open it, but can put another lock on the box.
B sends this box with the 2 locks back to A, A unlock his lock and send it back to B again.
So there is just B’s lock on the box and B can now open it.