The penny drops
Definition 1 of 1
Pro Tip 1/3Aha Moment
The phrase often describes a moment of sudden realization after confusion or misunderstanding. 
I couldn't understand the riddle, but then the penny dropped, and it all made sense.
Pro Tip 2/3Casual Expression
Common in casual, conversational contexts. Can sound out of place in very formal or serious situations. 
She chuckled when the penny dropped and I understood her prank.
Pro Tip 3/3British Origin
Primarily used in British and Australian English, may not be as widely recognized by American English speakers. 
Our American friend looked puzzled until I said 'I get it!' and then the penny dropped.