The "postcard -> unencrypted email as letter-in-envelope -> encrypted email" seems to be a very good analogy for explaining the need for encryption to laypeople

The "postcard -> unencrypted email as letter-in-envelope -> encrypted email" seems to be a very good analogy for explaining the need for encryption to laypeople. The technicals are trickier (and outside my scope of expertise), but the "why do I need to use this" answer is one of the trickiest and most important questions to answer for any security application.