When pigs fly 和 pigs might fly 為同義成語，意為「天方夜譚；絕不可能發生的事」，通常用來回應假設性或誇張的談話，帶有挖苦嘲弄意味。When pigs fly 為美式英語，而 pigs might fly 為英式英語。
Pigs might fly 的原始版本為 “Pigs fly in the air with their tails forward”，首次出現在英國辭典編纂者約翰˙威索爾斯 (John Withals) 1616 年出版的拉丁文-英文詞典《A Shorte Dictionarie for Yonge Begynners》中：”Pigs fly in the ayre with their tayles forward”。
例句：
- A: If we could just get the ruling party and the opposition to agree on a tax reform bill, we could bring the deficit down in no time. (A：如果我們能讓執政黨和反對黨就稅改法案達成協議，我們可以立即降低赤字)
B: Yeah, and when pigs fly! (B：是啊，但這是天方夜譚！) - A: You might get into the basketball team if you practice hard. (A：如果你努力練習，你可能進入籃球隊)
B: Yes, and pigs might fly! (B：是啊，但那是絕不可能發生的事！) - A: There’s a good chance (that) she’ll have her essay finished by tomorrow, of course. (A：當然，她很可能在明天之前寫完她的文章)
B: And pigs might fly. (B：天方夜譚) - A: Maybe one day we’ll be rich. (A：或許有一天我們會很有錢)
B: Oh look, I just saw a pig fly by my window. (B：哦瞧，我剛看到一隻豬飛過我的窗戶)