"After promising the inhabitants of Vrndavana that He would return very soon, Krsna underwent ritualistic ceremonies as a ksatriya. He took the vow of brahmacarya and lived in the guru-kula, where He studied regularly. By killing the demon named Pancajana, He received a conchshell named Pancajanya. Krishna rescued the son of His guru from the custody of Yamaraja and returned him. After thus offering guru-daksina to repay His teacher, Lord Krsna returned to Mathura-puri."
Srimad Bhagavatam, 10th Canto Summary
"Lord Krsna and Balarama, the reservoir of all knowledge of arts and sciences, exhibited Their perfect understanding when They offered to serve Their teacher by awarding him anything he desired. This offering by the student to the teacher or spiritual master is called guru-daksina. It is essential that a student satisfy the teacher in return for any learning received, either material or spiritual. When Krsna and Balarama offered Their service in this way, the teacher, Sandipani Muni, thought it wise to ask Them for something extraordinary, something which no common student could offer. He therefore consulted with his wife about what to ask from Them. They had already seen the extraordinary potencies of Krsna and Balarama and could understand that these two boys were the Supreme Personality of Godhead. They decided to ask for the return of their son, who had drowned in the ocean on the bank of Prabhasaksetra.
When Krsna and Balarama heard from Their teacher about the death of his son on the bank of Prabhasaksetra, They immediately started for the ocean on Their chariot. Reaching the beach, They asked the controlling deity of the ocean to return the son of Their teacher. The ocean deity immediately appeared before the Lord and offered Him all respectful obeisances with great humility.
The Lord said, "Some time back you caused the drowning of the son of Our teacher. I order you to return him."
The ocean deity replied, "The boy was not actually taken by me but was captured by a demon named Pancajana. This great demon generally remains deep in the water in the shape of a conchshell. The son of Your teacher might be within the belly of the demon, having been devoured by him."
On hearing this, Krsna dove deep into the water and caught hold of the demon Pancajana. He killed him on the spot, but could not find the son of His teacher within his belly. Therefore He took the demon's dead body (in the shape of a conchshell) and returned to His chariot on the beach of Prabhasaksetra. From there He started for Samyamani, the residence of Yamaraja, the superintendent of death. Accompanied by His elder brother Balarama, who is also known as Halayudha, Krsna arrived there and blew on His conchshell.
Hearing the vibration, Yamaraja appeared and received Sri Krsna with all respectful obeisances. Yamaraja could understand who Krsna and Balarama were, and therefore he immediately offered his humble service to the Lord. Krsna had appeared on the surface of the earth as an ordinary human being, but actually Krsna and Balarama are the Supersoul living within the heart of every living entity. They are Visnu Himself, but were playing just like ordinary human boys. As Yamaraja offered his services to the Lord, Sri Krsna asked him to return His teacher's son, who had come to him as a result of his work. "Considering My ruling as supreme," said Krsna, "you should immediately return the son of My teacher."
Yamaraja returned the boy to the Supreme Personality of Godhead, and Krsna and Balarama brought him to his father. The brothers asked if Their teacher had anything more to ask from Them, but he replied, "My dear sons, You have done enough for me. I am now completely satisfied. What further want can there be for a man who has disciples like You? My dear boys, You can go home. These glorious acts of Yours will always be renowned all over the world. You are above all blessing, yet it is my duty to bless You. I give You the benediction that whatever You speak will remain as eternally fresh as the instruction of the Vedas. Your teachings will not only be honored within this universe or in this millennium but in all places and ages and will remain increasingly new and important." Due to this benediction from His teacher, Lord Krsna's Bhagavad-gita is ever increasingly fresh and is not only renowned within this universe but in other planets and in other universes also."
Krsna Book, Chapter 45
Bhaktivedanta Book Trust. Excerpted from text and purport of HDG A.C. Bhaktivedanta Swami Srila Prabhupada.