Answer:
I am commonly abbreviated to H
Until I am measured, I am not known. Yet, how you miss me when I have flown. What Am I?
Weight in my belly trees on my back nails in my ribs feet I do lack.
Take it and you will lose or gain more than the others.
I am the book one can never finish reading
Almost everyone needs it, asks for it, gives it. But almost nobody takes it.
What am I?
Animal
Difficult
Easy
Food
Funny
Kids
Logic