砍不完的難題
把那條規則從故事裡拿出來，貼到現實的問題上。重點不是問題大，而是問題會長回來：你關掉一個假帳號，隔天冒出三個。
noun · C1
A problem that is very hard to get rid of, because every time you deal with one part of it, it comes back in another form.
打不完、一再冒出來的難題
核心畫面
One way to hold this word: you cut off a head and two grow back. That single rule is the whole word. It explains why the creature in the story is famous, why a monster type in games copies it, why a tiny pond animal that buds new bodies got the name, and why a problem that returns every time you solve it is called this. If a sentence does not have something coming back, this is probably not the word.
可以想成一個規則：砍掉一顆頭，長回來兩顆。整個字就靠這一條規則撐著。它解釋了為什麼神話裡那隻怪物有名，為什麼遊戲裡的…更多收起
可以想成一個規則：砍掉一顆頭，長回來兩顆。整個字就靠這一條規則撐著。它解釋了為什麼神話裡那隻怪物有名，為什麼遊戲裡的多頭怪照抄牠，為什麼一種會從身上長出新個體的小水生動物用牠命名，也解釋了為什麼「解決一個又冒出一個」的難題叫這個名字。句子裡如果沒有什麼東西一直長回來，多半就不是這個字。From the word’s history
The source traces the name to Latin Hydra, from Ancient Greek húdra, from a root meaning aquatic, and notes it is a doublet of otter. So the oldest layer is simply a water creature; the many heads come from the story told about it.
來源說這個名字來自拉丁文 Hydra，再往前是古希臘文 ὕδρα（húdra），更早的字根意思是「跟水有關的」，而且跟 otter（水獺）同源。所以最底層只是「住在水裡的東西」。那些頭，是後來故事加上去的。
Wiktionary (kaikki) entries for Hydra and hydra, supplied in packet: 「From Latin Hydra... from Ancient Greek ὕδρα (húdra), from Proto-Indo-European *údreh₂ (aquatic). Doublet of otter.」
From the picture
拿牠命名的真實東西
這一族不是比喻，而是借名字。科學家和觀星的人看到某個東西像那條蛇，就把牠的名字放上去，所以每一個都各借走畫面的一半。
noun · C1
A very small freshwater animal with a bare tube-shaped body and a ring of tentacles around its mouth. It belongs to the genus Hydra and to related groups.
水螅
Also in this word
noun · C1
In old Greek stories, the Hydra is a huge water snake with many heads. When one head is cut off, new heads grow in its place, so it is very hard to kill. A second source records that Hercules killed it as one of his twelve labours.
（希臘神話）九頭蛇
noun · B2
In games and fantasy stories, a dragon-like creature with many heads that can grow them back when they are cut off.
（奇幻作品）多頭怪