那張紙
畫面裡被手拿著的那個東西。它是材料，所以通常不可數，要數就用 a piece / a sheet；顆粒的粗細也是在講這張紙本身。
noun · B1
Strong paper with a rough, sandy surface on one side. You rub it on wood or metal to take off the rough parts and leave the surface smooth.
砂紙
核心畫面
One way to hold this word: a sheet of paper with sand stuck to one side, and a hand moving it back and forth over wood. The high bits come off, the dust falls, and what is left is smooth. The two halves of the name say it all — sand for the rough grains, paper for the thing that carries them. Everything else the word does comes from that one picture: the sheet itself, and the rubbing.
可以想成：一張紙，其中一面故意黏上沙；一隻手拿著它，在木頭上來回擦。突起的地方被磨掉，粉塵掉下來，剩下的就是光滑的面…更多收起
可以想成：一張紙，其中一面故意黏上沙；一隻手拿著它，在木頭上來回擦。突起的地方被磨掉，粉塵掉下來，剩下的就是光滑的面。這個字的兩半已經把畫面講完了——sand 是粗粗的顆粒，paper 是拿來承載顆粒的那張紙。它後來的各種用法，都可以看成這個畫面的兩個側面：一個是那張紙本身，一個是那個來回磨的動作。連粗細的講法也在這個畫面裡：先用粗的削掉，再用細的收尾。From the word’s history
From sand plus paper: paper with sand on it. The grains on modern sandpaper are usually made in a factory rather than taken from a beach, but the name kept the original picture.
就是 sand（沙）加 paper（紙）組成的。早期真的是把沙黏在紙上，所以名字直接照著東西叫。今天上面多半換成人造的磨料，名字卻沒有跟著改——那個「沙黏在紙上」的畫面，就這樣留在字裡。
Wiktionary entry for sandpaper, etymology_text 'From sand + paper.' supplied in this packet (source_ref kaikki:3f6f69a190036ae4:1175158703:30840)
Two branches
那個來回磨的動作
同一個畫面，鏡頭從紙移到手。英文常把工具的名字直接拿來當動作用，這一支就是這樣長出來的；連沒有真的拿砂紙、但結果一樣的情況也能用。
verb · B2
To rub something with sandpaper so that its surface becomes smooth. It is also used when something is worn smooth as if sandpaper had been rubbed on it.
用砂紙磨