Long, long ago, a baby boy was born to Kesari, the king of the monkeys, and Mata Anjana. Pavan Dev, the god of the wind, blessed him like his own son, so he is also called Pavanputra, the son of the Wind. This baby grew up to be Hanuman ji.
One morning, Mata Anjana went into the forest to bring fruit. The baby woke up very hungry. In the eastern sky he saw the rising sun, round and red. "Wow! What a big, sweet fruit!" he thought. With a happy jump, he flew up into the sky towards it. Pavan Dev flew beside him as a cool breeze, so that the sun's heat would not hurt him.
All the gods were amazed. Such a tiny child, and such a mighty leap! That day was an eclipse day, and Rahu was also coming towards the sun. The baby thought Rahu was a fruit too, and reached for him. Rahu rushed off to Indra Dev, the king of the gods. Indra came riding his white elephant, Airavat. And the baby thought the white elephant looked like a big fruit too!
Then Indra threw his vajra, his thunderbolt. It struck the baby's chin, and he fell down onto a mountain and lay still. In Sanskrit, the chin or jaw is called "hanu". That is how he got the name Hanuman.
Pavan Dev was heartbroken to see his son like this. He carried the baby into a cave and stopped the wind from blowing. All over the world, it became hard to breathe. All the gods came to the cave with Brahma ji. Brahma ji gently touched the baby, and little Hanuman opened his eyes. Pavan Dev smiled, and the cool breeze began to blow again.
Then every god gave Hanuman a blessing. Indra said the thunderbolt would never hurt him again. Surya Dev gave him a part of his own brightness and said, "When he is older, I will teach him the scriptures." Brahma ji and the other gods blessed him with strength, wisdom and a long life.
As he grew up, naughty Hanuman played many pranks in the ashrams of the rishis. So the rishis said, "For some time you will forget how strong you are. When someone reminds you, you will remember it all." And many years later, when it was time to help Shri Ram, Jambavan ji reminded him of his strength.
The lesson Strength is a gift from God; use it wisely and gently.
Source: Valmiki Ramayana, Kishkindha Kanda 66 and Uttara Kanda 35–36 (retold)
