"Fire is greater!" said Agni.
"Water is greater!" said Varuna.
They decided to have a contest to see who was right.
The God of Fire burned trees and crops and villages, but Varuna rained down and put out the fire. The God of Fire then fled into the mountain rocks, but the Rain kept pouring down.
Even now, Agni is hiding in the rocks; that's why when you strike rock with steel, sparks fly and you can make fire.
Inspired by: Heeramma and Venkataswami, or Folktales From India by M. N. Venkataswami
Notes: You can read more about the gods Agni and about Varuna at Wikipedia.