What does the idiom the day pigs fly mean

Based on this chart, how many details should each event contain on the time-order chart? (site 1)

The reversalists continue their labors, confident that they will one day construct an engine that generates more compression than it uses, a perpetual power source that will restore to the universe its lost vigor. which theme is best supported by the excerpt? a. scientific study requires dedication and perseverance. b. people will always look for a way to achieve immortality. c. death must always be endured alone. d. change is difficult to accept but beneficial if embraced.

Which sentence uses an antonym clue to the reader determine the meaning of the underlined word