1. "Once in a blue moon": This idiom is used to describe something that happens very rarely or infrequently. In the book, it is used to describe the rare occasions when the people of Ember are able to see the outside world.
2. "A drop in the bucket": This idiom is used to describe something that is very small or insignificant compared to something else. In the book, it is used to describe the amount of food that the people of Ember are able to produce on their own.
3. "At wit's end": This idiom is used to describe someone who is completely stumped or confused. In the book, it is used to describe the people of Ember when they are trying to figure out how to escape the city.
4. "A ray of hope": This idiom is used to describe something that gives hope or optimism. In the book, it is used to describe the discovery of a map that may lead to the outside world.
5. "A shot in the dark": This idiom is used to describe something that is done with little chance of success. In the book, it is used to describe the people of Ember's attempt to escape the city.