Explaining RSA to the people
How about this (and, at this level, it's not specific to RSA, but is equally analogous to any public key encryption method):


Max makes a safe with a slit on the top.  Max made the safe, so he knows the combination, but no one else does.  He gives the safe to Sam, who gives it to Dale.  Dale writes his note, and inserts it into the safe.  He gives the safe back to Sam.  Now, Sam can't open the safe, so he gives it to Max.  Max uses his combination to open the safe and read the note.


The nice thing about this analogy is that it captures some of the real concerns from a straight-forward use of public key crypto:


How does Dale know that the safe he was handed was actually made by Max?  That's quite valid, and is also a concern with public key cryptography.
What if Sam inserts his own message into the safe?  That's also a valid concern with public key crypto; as designed, Max has no way of knowing who wrote the message.


Both of these can be addressed, however that's a deeper topic.

(And, I'm pretty sure I read this somewhere, but I don't remember where...)