Leaven can be pictured as a pinch of something alive dropped into a big, heavy lump of dough. You can hardly see it, and yet it does not stay where it was put: it works out through the whole mass, filling it with air. What was dense and flat becomes light and full, and there is no part of the dough it has not reached. Two things in that picture are doing the work — a very small amount changes a very large thing, and the change goes everywhere.
From the word’s history
The etymology tree in our source traces leaven through Middle English levayn and Old French levain back to Vulgar Latin *levamen, from Latin levo 'to raise' and levis 'light'. The root idea of lifting and lightening sits under every use of the word.
我們手上的字源樹把 leaven 一路追到中古英文 levayn、古法文 levain，再到通俗拉丁文 *levāmen，源頭是拉丁文 levō（舉起）和 levis（輕）。「抬起來、變輕」這個根，正好撐著這個字所有的用法。
Wiktionary, "leaven"
登入就可以看每一支怎麼連回核心畫面。