Answer:
Two brothers we are, great burdens we bear. All day we are bitterly pressed. Yet this I will say, we are full all the day, and empty when go to rest.
What two things can always see what the other sees, but can never see each other?
What is given but kept by the giver?
What is eaten but not grown and was born in water but will disappear if soaked?
What has a mouth but can't chew?
The more of me you take, the more I appear behind you. What am I?