Nanda Maharaja and the other cowherd men who had come to Mathura from Vrindavana returned to their village.
Vasudeva, Krishna's and Balarama's father, wanted his sons to go to school. So They were sent to the gurukula of Sandipani Muni. Here, the two brothers learnt sixty four subjects in sixty four days!
They were so clever that They could understand and remember anything that was taught to Them just once!
After They had learnt all that Sandipani Muni had to teach them, Krishna and Balarama asked Their teacher what They could give him as guru-dakshina.
In those days, teachers did not collect money as fees from their students. They would be gifted various things that they needed in their day-to-day life like grains, clothes, jewellery and so on.
Sandipani Muni replied to Krishna's request, "I had a son who drowned in the ocean. Please find him and bring him back to us. My wife and I miss him so much. You are both very strong and smart boys. We are sure You can do this for us."
Krishna and Balarama set out for Prabhasa-kshetra, the place where Their teacher's son had drowned.
There, They ordered the ocean god to return the boy. The ocean god appeared before Them and said, "I did not take this boy. He was taken away by Panchajana, a demon who lives in the ocean. He may have swallowed the boy. This demon can be found in the deepest part of the ocean."