Take 'interesting' and add -ly, and you get 'in an interesting way'. That is the plain picture. From it the word grows a second job: instead of describing how one action was done, it steps to the front of the sentence and says something about the whole fact — worth noticing, this one.
可以想成 interesting 加上 -ly，先得到最直白的一個意思：以有趣的方式。這是這個字的原點。從這裡它又長出第二份工作：不再描述某個動作怎麼做，而是走到句首，對整句話講一句話——這件事值得注意。分辨的方法很實用：能換成 in an interesting way 的，是原點那個用法；換不通、而且刪掉之後句子的事實完全不變的，就是站在句首下註解的那個。
From the word’s history
Recorded as interesting + -ly, with -ly forming the ordinary manner adverb; the sentence-adverb use sits on top of that.
字典把它記成 interesting 加 -ly。-ly 原本的工作是造出「以某種方式」的副詞，而句首評論的那個用法是後來疊在上面的第二層。
Wiktionary 'interestingly' etymology tree (supplied in evidence packet, kaikki:3f6f69a190036ae4:1196180722)
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