Answer:
Goes over all the hills and hollows, Bites hard, but never swallows.
I am in the past, never in the future. I don't exist, but have existed. I saw what you saw, and this is what I will ever see. What am I?
I go around in circles, but always straight ahead, never complain, no matter where I am led.
I have thirteen hearts but no body or soul.
Why is it so easy to weigh fish?
No matter how little or how much you use me, you change me every month.