Think of a small machine that goes tick … tick … tick, one short beat for every single thing that happens. Each beat leaves a mark behind: a price printed on a paper strip, a line sliding across a screen, a beat inside your chest, a check drawn beside a name on a list. The word ticker points at whatever is doing that steady, one-at-a-time marking — never at a finished, still picture of everything at once.
The packet's source entry says the word is simply tick + -er: the thing (or person) that ticks. English tick covers both the small repeated sound and the small written mark, and both of those roads are used by this word.
資料上寫 ticker 就是 tick 加上 -er：做 tick 這件事的東西或人。英文 tick 同時有「滴答聲」和「打勾」兩個意思，這個字兩條路都走了。
Wiktionary entry for ticker, supplied in this packet: "From tick + -er."
Three branches
會報數的機器，和它吐出來的那一條
這是整個字最原始的樣子：一台每收到一筆資料就響一下的機器。核心畫面裡的「一下一下、一件一件」在這裡是看得到、聽得到的。
noun · B2
A machine or screen that shows share prices one after another as they rise and fall during trading. The older kind printed each price onto a long, thin strip of paper.
股價機、股價跑馬燈
胸口裡那台一直在響的機器
把核心畫面整台搬進身體裡：心臟也是一下一下地跳、從不停下來的那一個，所以借用同一個字，語氣輕鬆。
noun · B2
Your heart. This is a warm, informal word, used most often when people talk about whether someone's heart is strong or weak.
心臟
一項一項打勾的人
這一支走的是 tick 的另一個意思——在清單上打一個勾。核心畫面裡「一件一件記下來」仍然成立，只是聲音換成了筆尖的那一個勾。
noun · C1
Someone whose hobby is to see or try as many different things as possible and mark each one off a list. It is used most about birdwatchers, and in the same way about people who record every different beer they have tasted.
逐項打勾的收集者