pick a direction and travel some distance in that direction
We'll say north and one hundred miles, for simplicities sake.
Once you have finished your one hundred miles
turn 90 degrees to your right or left and travel one hundred miles
once you have completed that, once again turn 90 degrees (This time to the opposite of your last turn)
and travel one hundred miles
what is your location?
if you guessed, one hundred miles to the east or west of my start point (depending upon which your first turn was)
you would be correct
however in a spherical model
you would arrive back at your point of departure.
(since you don't enjoy being fun I'll note I don't actually believe this, and this model expanded to a larger number of miles in each direction would have you arriving at your start point, actually theoretically you could travel any number of miles after your first turn and it wouldn't matter.)