Hey /r/boardgames! Do you find yourself carrying around a certain boardgame in case of spontaneous gameplay?

When I go to the local university I always bring my backpack in case I need pens or paper or other school related things, but I also carry around Red Dragon Inn and a couple expansion packs. The other day I was just about to head home from the university, when a fellow boardgamer stopped me and told me he had organized a boardgame night with his friends there and asked me if I would like to join, and conveniently enough I had Red Dragon Inn. I taught him and his other friends how to play and a good time was had by all. I carry around Red Dragon Inn because it's easy to learn, has a ridiculously fun concept, and with all the expansions you can make it a 14 player game if you choose to do so. Do you carry around a boardgame in case of a crazy random happenstance? If not, what game would you keep with you at all times and why?