The first of Ofelia's three tasks is to retrieve a key from the stomach of a giant toad.
The toad lives inside of a tree and lives off of bugs. He eats all the bugs and gets fat, as Ofelia put it, whilst the tree dies. When confronting the toad, Ofelia tricks it into eating magical stones given to her by the Faun that happen to look a lot like bugs. As a result, the toad vomits up the key along with its fat and several vital organs. The toad dies and Ofelia gets the key. All is well... for now.