The penny's dropped

[brisey]

Where does the phrase 'the penny's dropped' come from?

[tonyb]

This phrase, which signifies when someone finally realises something, comes from penny-in-the-slot machines, in which the penny has to drop to make then work.

[kermy]

Years ago, in old seaside piers and arcades, one would put a penny into a fortune teller machine, an animated head would go round and a card with an answer on it would come out of the bottom, hence the wisdom was told once the penny had dropped into the machine

[cyclefrance]

My first thoughts are that it could relate to early UK public phone boxes (the old red ones). You'd lift the handset, put a penny in the slot and dial the number. When the phone was answered, you had the choice of pressing button A or button B. Pressing B gave you your money back - so if you got a wrong number you didn't have to pay (so long as the other person gave you a clue that it wasn't the right one), an the person receiving the call had no clue as to who had been callng them.. Pressing button A caused the penny to drop and the person at the other end could then hear who was calling them. The identity mystery was solved, but only when button A was pressed, the penny dropped (you could actually hear it clank into the box below the slot), and the caller could be heard at the other end.