A tracer is a bullet that burns as it flies, so it leaves a line of light or smoke in the air behind it. Soldiers load a few of them in among ordinary bullets — every fifth one, say — so that at night, or when they are shooting far away, they can see where their shots are going and correct their aim.

A tracer is曳光彈是 a bullet that burns as it flies一種飛行時會燃燒的子彈, so it leaves因此會留下 a line of light or smoke in the air behind it一道光或煙的痕跡在它身後的空中。Soldiers load a few of them in among ordinary bullets士兵會把幾發曳光彈混在普通子彈裡, every fifth one, say比如說每五發裡放一發, so that at night, or when they are shooting far away這樣一來在夜間，或是射擊距離很遠的時候, they can see where their shots are going and correct their aim他們就能看到子彈的落點，修正瞄準。

The word is also used for the burning line itself: people talk about tracers going up into the sky. You will meet the word mostly in war films, war reports, and video games, and it is often used as 'tracer fire' or 'tracer rounds' rather than alone.

The word is also used for這個字也可以用來指 the burning line itself那道燃燒的痕跡本身: people talk about人們會說 tracers going up into the sky曳光彈射向天空。You will meet the word mostly in你最常會在 war films, war reports, and video games戰爭片、戰爭報導和電玩裡遇到這個字, and it is often used as而且它常常寫成 'tracer fire' or 'tracer rounds' rather than alone「tracer fire」或「tracer rounds」，而不是單獨使用。

Outside guns, the same idea gives 'tracer' another life in hospitals: doctors put a small amount of a substance into your body that a machine can follow, and that is also called a tracer.

Outside guns在槍砲之外, the same idea gives 'tracer' another life in hospitals同樣的概念讓 tracer 在醫院裡有另一個意思: doctors put醫生會放入 a small amount of a substance into your body一小份物質到你的身體裡, that a machine can follow讓機器可以追蹤它, and that is also called a tracer這也叫做 tracer（示蹤劑）。

The thread running through all of it is the plain verb 'trace' — to follow a line, or to leave one that can be followed.

The thread running through all of it貫穿這一切的主線 is the plain verb 'trace'就是最簡單的動詞 trace──to follow a line, or to leave one that can be followed意思是「循著一條線走」，或「留下一條可以被循著走的線」。