On a whim, you packed your bags and headed to a new city for a weekend getaway. No map, no plan; just a car, an open road and a vague desire to see the sights. You turn left, then right, then straight ...

Have you ever played the 21 Game? It’s a deceivingly easy numbers game, which goes like this: Starting at 0, two players take turns adding 1, 2, or 3. For instance, the first player could start by ...