One day the powerful sage Parnada was cutting some grass. When he cut his finger, instead of blood, his finger oozed the sap of a tree.
He was filled with pride. Shiva who witnessed this event took the guise of an old man and asked him the reason for his delight, and Parnada said that he had become the most pious man in the world.
But the old man questioned the sage's joy saying that it was just sap; when trees and plants are burned they turn into ash.
The old man demonstrated it by slicing his finger, which caused ash to spill. Sage Parnada realized that it was Shiva before him and asked him to forgive his ignorance.