Agni, the fire god in Hinduism, was cursed to consume anything that is pure or impure. Brahma intervened and changed that curse. This story found in Kashidasi Mahabharat is based on the belief that in the beginning Agni only accepted pure things like soma, ghee etc.
A demon once arrived at the ashram of Sage Bhrigu when he had gone out for his morning ablutions. Puloma, the pregnant wife of Sage Bhrigu, was alone at home and she only had the Agni in kitchen as her protector.
The demon claimed that Puloma’s father had promised her in marriage to him. But her father did not keep the promise and married her to Sage Bhrigu. Agni was the witness to all this the demon claimed.
Agni was not sure so he gave a diplomatic answer. He told that may be her father might have promised. But he is only sure about the fact that Puloma is the wife of Sage Bhrigu as they had gone seven times around the fire as part of marriage.
Puloma was angry at Agni for not completely favoring her.
The demon tried to kidnap Puloma. But in fear she gave birth to a boy. The boy looked angrily at the demon harming his mother. The demon was burned into ashes.
Puloma narrated the incident when Sage Bhrigu returned.
Sage Bhrigu was angry at Agni not protecting his wife. He cursed Agni that from then onward to consume all that is pure and impure.
Agni thought the curse was against Dharma as he had nothing to do with the whole episode and he did not say anything against Puloma.
Lord Brahma intervened and told Agni that whatever he touched will become pure in future. He told that all impurities will burn away when touched by Agni.