le franc est tombé

to finally understand something

Said the moment the penny drops and something finally clicks.

Je lui ai expliqué trois fois, et puis d'un coup, le franc est tombé.

I explained it to him three times, and then all of a sudden, the penny dropped.

Comes from old coin-operated vending machines, where nothing happens until the coin drops and the machine releases your coffee or soup. Belgians borrowed that click of understanding for the franc, and some now joke about the euro dropping instead.