Are there any similar phrases that are popular in the US to express "penny dropped"?
- light-bulb moment
A moment of sudden realization, enlightenment, or inspiration
[ODO]
- And then, it hit me!
That exact moment I finally understood something.
[Urban Dictionary] (for lack of better resources)
You could say that it clicked.
informal to become suddenly clear: it finally clicked when her name was mentioned.
[AHD via TFD]