Hot irons leave marks on what they touch, if they stay in that place for some time. In the old American west, branding irons were used to mark the hip of an animal to show that the animal belonged to a certain person. Today, if a person wants to do something at the best time, they do it when the iron (idea) is hot (ready to use).
我們決定打鐵趁熱（馬上做）。
從前鐵匠以手工打鐵，須把鐵塊放入爐中燒紅，取出後立即捶打；否則鐵冷了、變硬了，就打不動了。對於「打鐵趁熱」的由來，另一種見解，請參考英文說明。