Answer:
A natural tattoo on babies
This is your stomach's way of letting you know you've neglected it.
If you need a night out, call one of these over to take the little ones off your hands for a while.
It is in every mountain, it's not in any hill, it's not in all the world, and yet it's in the mill.
They are Dark, and always on the run. without the sun, would be none!
People take me on walks and I run and have fun. What am I?
Easy
Animal
Difficult
Food
Funny
Kids
Logic
What am I?