This time in English: How to explain the principles of PGP, looking for metaphors

[Daniel Krebs]

Sorry!
I picked the wrong language / list last time...
So in English:
What metaphors do you use when explaining people PGP? Two examples:
1. A lock with two keys?
2. A lock (public) and a key (private)
Something completely different?

Problems with both:
1. Seems to be kind of hard to understand for most people, because a 
lock with one key to open and one key to close is rather special.
2. Signing emails is hard to explain this way. Signining by putting a 
lock on it?

Any ideas are appreciated.

An Interesting approach (Thanks Neal for the link): Using 4 items: key, 
lock, seal and imprint.
https://freedom-to-tinker.com/blog/randomwalker/why-king-george-iii-can-encrypt/


-- 
kind regards
daniel krebs