Acid is a substance that destroys other substances in contact with it.
It dissolves every element except Glass, Multi, and Block. Metal has a resistance to it, but breaks down after repeated contact. Metal becomes Electrified Metal when touched with Acid.
Acid can also combine with Water, and form Acid Rain. Acid sinks through acid rain, but water doesn't. So to create large amounts of acid rain, the acid should be poured on water.