a light bulb goes off in (one's) head(redirected from light bulb goes off in their heads)
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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.