Answer:
Reaching stiffly for the sky, I bare my fingers when its cold. In warmth I wear an emerald glove and in between I dress in gold.
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.
Looks like water, but it's heat. Sits on sand, lays on concrete. A play on the eyes, but it's all lies.
A hole leading in, a hole leading out, we connect to a cavern that is slimy all throughout. What are we?
Large as a mountain, small as a pea, Endlessly swimming in a water-less sea.
What is lighter than what it is made of?