Just take the time to explain the system properly.  Don't use metaphors - they all suck and you'll just confuse peole more.  Start with basic cryptography and then introduce public key crypto.  Don't go into detail, just give them the overview.  If they say silly things like 'that will never work' or 'it's unproven' just tell them that online banking and shopping have been using it sucessfully for 30 years.