Hanuman was on his way to Lanka when a hungry monster arose from the ocean. It opened its mouth wide to block Hanuman. Hanuman then pleaded to let him go, but the monster refused to listen.
Seeing no other way out, Hanuman first grew in size. When the monster too opened its mouth wide, he quickly shrank again and flew in and out of the monster's mouth. He said, "I have fulfilled your wish, now let me complete my mission."
This monster was actually the snake maiden. Hearing Hanuman's plea she changed into her original form and replied, "My son, I'm Surasa, the snake maiden. The gods had sent me to test you. Go ahead; your mission will be successful."
Overcoming many more dangers on the way, Hanuman continued on his way.