Answer:
You will find me with four legs, but no hair. People ride me for hours, but I don't go anywhere without needing to be tugged. Jerked or turned on, I always manage to be ready for work.
What has roots as nobody sees, is taller than trees. Up, up it goes, and yet never grows?
It has no top or bottom, but it can hold flesh, bones, and blood all at the same time.
I cost no money to use, or conscious effort to take part of. And as far as you can see, there is nothing to me. But without me, you are dead.
What do you fill with empty hands?
If itβs information you seek, come and see me. If itβs pairs of letters you need, I have consecutively three. What am I?