The penny dropped is an idiom that means someone finally understood something, after some confusion or delay. A similar idiom is a light bulb moment or to put two and two together.
It originates in older slot machines, where there was a delay between inserting a coin, and the mechanism starting to work.
It originates in older slot machines, where there was a delay between inserting a coin, and the mechanism starting to work.
She stared at the instructions for a full 10 minutes before the penny dropped.