a light bulb goes off in (one's) head

a light bulb goes off in (one's) head

One is suddenly struck with an ingenious or inspired idea, insight, or revelation. In this expression, the phrasal verb "go off" means "turn on" or "become active." We spent hours trying to come up with a solution, when suddenly a light bulb went off in my head. I kept feeding him subtle hints about the truth of the matter, waiting for a light bulb to go off in his head.
See also: bulb, goes, head, light, off