Can any of you geniuses out there help with a simple riddle that my buddy Jack and I are having trouble with , Jacks daughter came home from school with the riddle the other day .......When i am full i indicate directions and when i am empty i can,t be moved .......A few things that it is not are windsock , sail , car 
Could it be something a bit more abstract, like "A Vector" or something? (Assuming the riddle is from a Maths class)
Otherwise, Dude seems on the right track with something needing fuel.
