Answer:
I go around in circles, but always straight ahead. Never complain, no matter where I am led.
When set loose I fly away. Never so cursed as when I go astray.
What goes up but never comes down?
What is long and slim, works in light. Has but one eye, and an awful bite?
Different lights do make me strange, thus into different sizes I will change
You use it between your head and your toes, the more it works the thinner it grows.
Difficult
Animal
Easy
Food
Funny
Kids
Logic
What am I?