Answer:
I am long and slim works in light. Has but one eye and an awful bite.
I belong to you but others use me more often than you do. What am I?
I can be thin but not fat; in your body but never on your placemat. I'm always better when I'm fresh but you'll never see me in the flesh.
I carry you down a gentle stream.
I contain six letters, minus one and you got twelve. What am I?
No matter how little or how much you use me, you change me every month.