A lightbulb suddenly lit up in his mind when he came up with the solution.

lightbulb 是复合名词，由 light（光）和 bulb（灯泡）组成，字面意思是"发光的灯泡"。 在英语中常用"a lightbulb lit up in one's mind"这个比喻表达，表示"突然想到好主意"，就像灯泡突然亮起一样形象生动。