Answer:
I go in hard, come out soft. Blow me hard and I'll make a pop. What am I?
I tremble at each breath of air, and yet can heaviest burdens bear. What am I?
I'll bring out your bulls and slow your roll. I may even come out of your skull.
What has a head and a tail but no body?
I help engines spin and pants stay up.
No matter how little or how much you use me, you change me every month.