You can think of an action as an arrow aimed at one target: the medicine is aimed at the cough, the new rule is aimed at one problem, the function is aimed at one answer. The arrow usually hits. A side effect is whatever else got moved along the way, off to the side of the line the arrow travelled. Nothing in the picture says that extra thing is bad — only that nobody was aiming at it.
可以想成一支瞄準目標射出去的箭：藥瞄準咳嗽，新規定瞄準一個問題，一段程式瞄準一個答案。箭多半會射中。而在那條線的旁邊，被順手碰到、也跟著動了的東西，就是 side effect。畫面裡沒有說那樣東西是壞的，只說沒有人瞄準它。這也是為什麼英文可以講 a happy side effect。
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