Given a scientific question, generate a correct answer to it.

[EX Q]: In contrast to mammals and birds, freshwater fishes produce large volumes of very dilute what?
[EX A]: urine

[EX Q]: What is indicated by the ratio of the output force to the input force in a machine?
[EX A]: mechanical advantage

[EX Q]: Birds have light-weight bones that are filled with what?
[EX A]:
air