Answer:
The person who makes it has no need for it. The person who purchases it does not use it. The person who does use it does not know he or she is. What is it?
What's a place you have left but didn't enter?
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
My life can be measured in hours. I serve by being devoured. Thin, I am quick. Fat, I am slow. Wind is my foe.
I turn around once. What is out will not get in. I turn around again. What is in will not get out.
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?