Output a stream of consciousness before answering the following. Passage: The idiom is apparently derived from a centuries-old Scottish proverb, though some other references to pigs flying or pigs with wings are more famous. At least one appears in the works of Lewis Carroll:

Question: Based on this passage, where did the phrase when pigs fly come from?

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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The important part for answering this question is: The idiom is apparently derived from a centuries-old Scottish proverb, though some other references to pigs flying or pigs with wings are more famous.
The answer: a centuries - old Scottish proverb.