How many places on Earth can you walk: 1 mile South, 1 mile West, and then 1 mile North, and end up in the same place where you started from?
For simplicity, assume that the Earth is a perfectly smooth sphere - so no one complains about hitting mountains or icebergs on their trip. And all the conventional properties of directions count, i.e. - moving South & North means parallel to longitude lines and West parallels a latitude line, etc.
For all you physicists, this means zero displacement, but a distance of 3 miles traveled.
If anyone has already seen this riddle, don't spoil it for the others!