液體上的泡沫
這一組就是核心畫面本身。同一件事可以從三個角度說：泡沫這個東西（名詞）、液體自己冒泡（不及物動詞）、有人把液體打出泡（及物動詞）。
noun · B2
A soft mass of very small bubbles sitting on top of a liquid — the white layer you see on hot milk, on a shaken drink, on soapy water, or on a breaking wave.
泡沫
核心畫面
Air gets into a liquid, tiny bubbles rise, and they pack together into a soft white layer on the surface. Three things about that layer travel into every other use of the word: it sits on top of something else, it looks light and pretty, and it is gone in a minute. Whether the layer is on hot milk, on a wave, at the corner of a mouth, or on top of a market, the picture stays the same.
froth 可以想成液體表面那一層又細又白的小泡泡。空氣被打進液體裡，泡泡浮上來，擠成軟軟的一層。這層泡沫有三個特徵…更多收起
froth 可以想成液體表面那一層又細又白的小泡泡。空氣被打進液體裡，泡泡浮上來，擠成軟軟的一層。這層泡沫有三個特徵會一路帶到其他意思：它浮在最上面、看起來蓬鬆好看、而且很快就消掉。不管是熱牛奶上的奶泡、浪打碎後的白沫、嘴角的口沫，還是被追高的市場價格，畫面都是同一層浮在上面、留不久的泡。From the word’s history
English took froth from Old Norse froða, from a Germanic root that also gave Old English āfrēoþan, meaning to foam. It has named bubbles on a liquid from the start, and the verb is recorded from the late 14th century.
froth 來自古諾斯語 froða（泡沫），同一個日耳曼語的根也給了古英語 āfrēoþan（起泡）。它從一開始講的就是液體上的泡泡，動詞用法在十四世紀後期就有紀錄。所以現在這些意思，都是從最原始那層泡沫延伸出去的。
Wiktionary entry for froth (packet source_ref kaikki:3f6f69a190036ae4)
Three branches
嘴邊的白沫
同一層白泡，位置從杯口換到嘴角。口水被空氣打散成細泡，看起來就跟液體上的泡沫一樣。因為這種樣子通常代表身體出狀況，這一組帶著「不對勁」的味道；最後再從這個畫面誇張成「氣到失控」。
verb · C1
If a person or an animal froths, white bubbles of saliva come out of their mouth, usually because they are ill or having a fit.
口吐白沫
像泡沫的內容與價格
這一組借的是泡沫的另外兩個特徵：浮在真東西的上面，而且很快就消掉。用在話語上，就是好聽卻沒有實質；用在市場上，就是價格裡那一段沒有實際價值撐著的部分。
noun · C1
Talk, writing, or ideas that are pleasant and easy to enjoy but hold nothing solid, useful, or true underneath.
空話；華而不實的內容