Answer:
What is it that given one, you'll have either two or none?
You have it even if there is none.
This is a dead giveaway
What is used by man, tossed by trees, everywhere but unseen?
Sitting down you have it, Standing up you don't.
People take me on walks and I run and have fun. What am I?
Easy
Animal
Difficult
Food
Funny
Kids
Logic
What am I?