I am needed for most animals and hardcover books. What am I?

Answer: Spine

More What am I ? with answers

More What am I ?

I am not a toy, though many use me for pleasure. I have a butt but I can not poop. I have been known to kill people. What am I?
I am not a toy, though many use me for pleasure. I have a butt, but I cannot poop. What am I?
I am not alive and yet I grow. Just put me next to where it grows. A favorite of the summertime, best with friends when combined. What am I?
I am not alive but I can die. What am I?
I am not alive but I can grow. I do not have lungs but I need air to survive. What am I?
I am not alive but I grow. I don't have lungs but I need air. What am i?
I am not alive but seem so, because I dance and breathe with no legs or lungs of my own. What am I?
I am not alive, but I grow; I don't have lungs, but I need air; I don't have a mouth, but water kills me. What am I?
I am not very commonly found! Only in some rainforest! I have an odd number of toes! I’m very lazy and hang upside down! What am I?
I am often sold by children entrepreneurs in summer time. What am I?
I am one of many, You forget I'm here, but I'm just below. Without me, you'd surely topple, Go ahead, what am I?
I am one small little piece of paper, yet sometimes hold lots of value. I am all you need to get in to big events, but will cost you. I am an important part of travel. And if lost, you’re not coming. What am I?