One finally catches on to something. The term alludes to old-time machines in which one deposited a penny to obtain a piece of candy, which one received after the penny dropped all the way down. Dating from the 1950s, it is used more in Britain than in the United States but is still current. Alexander McCall Smith used it in The Right Attitude to Rain (2006): “I think he realises now that she’s never going to come back to him. It’s taken a long time, but the penny has finally dropped.”