There is an expression,the penny has dropped, that I became aware of recently and I had no idea what it meant. I know the expression,dropping a dime on someone (see below), but this was new to me. As in interested wordsmith, I went to Google to retrieve the answer.

It refers to someone realizing the situation they are currently in after it possibly eluded them for some time. You can find out the derivation of the phrase by checking out the link from the Urban Dictionary. Phrase evolution is very interesting indeed!

Dropping the Dime on Someone – At one time a telephone call cost ten cents. Dropping a dime refers to tattling on someone or revealing information about someone via a telephone call. Of course it no longer has to be via a telephone call. The phrase is not in popular contemporary use, but it is found in detective novels from time-to-time.