The gods, siddhas, and gandharvas requested Surasa, the mother of serpents, to test Hanuman's strength and intelligence by obstructing his way. Surasa emerged from the depths of the sea, assuming the form of a frightening rakshasi, and became a barrier in Hanuman's path.
"Today, the gods have sent me good food; I will devour you," Surasa declared to Hanuman. "You must enter my mouth if you wish to go forward."
"I am going to carry out Lord Rama's task," Hanuman replied, joining his hands in supplication. "I need to inquire about Sita. I will return to you once I have accomplished my task, and then you may swallow me alive."
"I have a boon from the gods by which no one can pass me without my permission. So, enter my mouth now!" demanded Surasa, opening her mouth wide and expanding it.
In response, Hanuman began increasing his size as well. Surasa's mouth grew to a hundred yojanas wide. Suddenly, Hanuman had an idea. He assumed a tiny form, quickly entered Surasa's gigantic mouth, and swiftly emerged.
"I have done as you wished," said Hanuman. "I entered your mouth and came out. Now allow me to proceed to inquire about Sita's whereabouts."
"Oh, Hanuman!" exclaimed Surasa, assuming her natural form. "I was only testing your strength and intelligence. You may go and do whatever you need to reunite Rama and Sita."