Think of a knife going straight across something and stopping there. What matters is not the piece you took off — it is the flat face now showing. From outside you saw only skin; one cut across, and every layer inside is visible at once, side by side.
可以想成一把刀橫著切過去，然後停在那裡。重點不是切下來的那一塊。重點是現在露出來的那一面。從外面看，你只看得到表皮。橫著切一刀，裡面每一層就同時攤在你眼前，一層挨著一層。這個「一刀下去、每一層都看得到」的畫面，就是 cross-section 兩個意思共用的那根線。
登入就可以看每一支怎麼連回核心畫面。