Here is how I explain public/private keypairs to new people:  
     I tell them you have a public address and a private key.  If you order something from Amazon you give them your public address (you never give them your house key) so they can deliver the goods.  Once they drop the package through the mail slot they can no longer access it or take it back because it's locked in your house that only your private key can open.  
     Same thing with Bitcoin, you have a public address/key that someone sends BTC to you at and once it's sent it is locked behind your private key that they don't have.  
     The security aspects is where it falls apart - trying to analogize that your private key also has your public address on it, so if someone gets your key they know where your house is and you can't stop them from taking your BTC.  They must never find your key because they can take your stuff from anywhere.  And that you must never lose your key because then you are locked out and there are no Bitcoin locksmiths.