Pippalada was an ancient Indian Vedic sage and philosopher in Hindu mythology. He is known to have written the Prashna Upanishad, which is among the ten mukhya(main) upanishads. He was an avatar of lord Shiva, and son of sage Dadhichi, who donated his bones to Devtas(Demigods) to provide them a material for making weapons and defeat the Asurs. He was the founder of Pippalada School of thought, which taught the Atharvaveda.
At the time his father Dadhichi gave up his life to donate his bones to [Indra] and help him defeat the asur Vritra, his wife Swarcha was pregnant with Pippalada. When Swarcha came to know about death of her husband, she cut her womb with a stone and placed the infant Pippalada under a pippal tree. Then she committed suicide to reunite with her husband. The infant Pippalada was taken care of by Dadhichi's sister, Dadhimati. He helped himself to the pippal leaves and amrit(nectar) given by Chandra dev(God of moon).
Revenge with Indra
When Pippalada grew up and came to know about the way his parents died, he performed a heavy penance to Lord Shiva and obtained a demon to kill all the devtas. All the devtas went to Shiva for protection, Shiva made Pippalada realize that nothing can be gained by killing the Devtas as it will not bring his parents back.
Pippalada defeated Shani. Shani to save his life promised that he will never trouble anyone below 16 years of age.
The following six sages: Kabandhin Katyayana, Bhargava Vaidarbhi, Kausalya Asvalayana, Sauryayanin Gargya, Saibya Satyakama and Sukesan Bharadvaja approached Pippalada asking him about purpose of life. They performed austerities for a year and asked six questions. These questions and answers later came to be known as the Prashna Upanishad.