Five. That's the magic number when traveling in Germany. It's the maximum number of people that can use one Schönes Wochenende ("Happy Weekend") pass, which can be used to board an unlimited number of regional trains for one entire day. It's the minimum number of people needed to get a group discount in the BMW Museum. It's also the optimal number of people to share a rental car. And that was how we were able to make a very hastily-planned weekend getaway to Germany's "most liveable city" for less than a hundred euros.