Sanda and Amarka were becoming desperate. They saw that their end was near. They questioned Prahlada as to how it was possible that they gave him poison and yet, he did not die. They tried to burn him in fire and it didn’t work. They even threw him in a snake pit to no avail.
Prahlada explained that poison was his uncle. His true father was Narayana and the Lord’s consort had appeared from the milk ocean, and prior to her poison had appeared from that same ocean, as her elder brother. Thus poison was Prahlada’s uncle and would not harm him. Fire was the mouth of Visnu for accepting oblations in sacrifice, and the Lord would certainly not harm his son Prahlada. Even when Prahlada was thrown in a pit with snakes, they did not bite him. Prahlada explained that His father, Lord Visnu was always resting on a bed of snakes, so when a child plays on His father’s bed, how can any harm come to him? – Vedanta Desika, a 12th Century Sri Vaisava told by Kripamaya Prabhu (courtesy of http://kkswami.com/)