Answer:
Forward Iโm heavy, but backwards Iโm not.
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.
In the night a mountain, in the morning a meadow.
I dig out tiny caves and store gold and silver in them. I also build bridges of silver and make crowns of gold. They are the smallest you could imagine. Sooner or later everybody needs my help. Yet many people are afraid to let me help them.
I drift forever with the current down these long canals theyโve made, Tame, yet wild, I run elusive Multitasking to your aid. Before I came, the world was darker, colder, sometimes, rougher, true. But though I might make living easy, Iโm good at killing people too.
If itโs information you seek, come and see me. If itโs pairs of letters you need, I have consecutively three. What am I?