Answer:
You need a key to receive an answer from me. The answer you'll find is straight from your mind. What is it?
What is up that’s also down? What’s a smile and a frown?
What can be swallowed, but can swallow you?
What grows in winter, dies in summer, and grows roots upward?
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.
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?