THE BAY AREA PASSWORD
I was playing a videogame recently where two characters have to identify each other as friendly forces by each repeating half of a code phrase; I'd remember what this is called, but this cold I've got has put a serious dampener on my ability to think quickly. In any case, it got me thinking about what would happen if the Bay Area were ever invaded, and what we could use to quickly identify ourselves to each other verbally as friendly forces and not invaders.
Then it hit me: you could just shout out "I'm at the Pizza Hut." If they don't respond with "I'm at the Taco Bell," you are clear to open fire.
I know this would only work for a subset of the local population, but those people are definitely the subset I want to make sure I don't shoot.