the penny drops
US /ðə ˈpɛni drɑps/
UK /ðə ˈpɛni drɑps/
1.
used to say that someone has finally understood something, especially after a period of not understanding
:
•
It took him a while, but then the penny dropped and he realized his mistake.
•
I kept explaining, but it wasn't until I drew a diagram that the penny dropped.