the penny drops
Now I (etc) understand
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Main Entry: ↑penny
* * *
the penny drops — see ↑penny
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Main Entry: ↑drop
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the penny drops
Brit informal
— used to say that someone finally understands something after not understanding it for a time
I had to explain it to him three times, but finally the penny dropped. [=he understood the explanation]
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Main Entry: ↑penny
Useful english dictionary. 2012.