Kesari was king of the monkeys, and his wife was Anjana. Vayu the Wind-God gave her a child, but she wandered off and left the baby alone.
The little baby grew hungry, and when he saw the sun overhead he thought it was a mango, so he flew up into the sky, wanting to eat the mango.
The gods were amazed at the power of Vayu's son! Indra the Storm-God grew angry and struck the baby with a thunderbolt. The baby then fell down to the ground and broke his jaw; that is how he got the name Hanuman, Big-Jaw.
Inspired by: The Ramayana (Uttara Kanda, 35).
Notes: The complete story involves the eclipse-demon Rahu who is angry that Hanuman might get to eat the sun, but I did not have room to include Rahu here.