mediumwhat am-i

When I am filled I can point the way, When I am empty Nothing moves me, I have two skins One without and one within. What am I?
Common Wrong Answers
“A compass”
While a compass does point the way when it is filled with a magnetic needle, it does not have 'two skins' as mentioned in the riddle.
“A bottle”
A bottle can point the way when filled with a liquid, but it does not have two skins and the concept of being empty does not relate to movement.
“A map”
A map can point the way when it is 'filled' with information, but it does not have physical skins, and being empty does not stop it from providing directions.