I'm posting one puzzle, riddle, math, or statistical problem a day. Try to answer each one and post your answers in the comments section. I'll post the answer the next day. Even if you have the same answer as someone else, feel free to put up your answer, too!
Sunday, October 16, 2005
Fill me up
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?