You can think of a glass that is half full. Nobody takes it away and nobody it. Someone simply pours in just enough to bring the level back up to the top, and that one pouring is the top-up. Two things are true in that picture and they stay true everywhere else the word goes: something was already there, and what you add is only the missing part. Change the glass into a phone account, a travel card, a loan, or an insurance policy, and the word still means the same small act of bringing the level back to where it should be.
可以想成一個喝了一半的杯子。沒有人把它拿走，也沒有人把它倒掉。有人只是再倒一些進去，把水位補回原本的高度——那一次倒的量就是 a top-up。這張圖裡有兩件事，在這個字的每一個用法裡都成立：本來就已經有一些東西在裡面，而你加進去的只是缺的那一段。把杯子換成手機帳戶、悠遊卡、一筆貸款、一張保單，這個字的意思都沒有變，還是那個「把水位補回該有的高度」的小動作。
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