the penney's dropped

[big yankee]

Why do we use this expression following a sudden awareness?

[Hippy]

Pennies used to be required to gain entrance to the cubicles in public toilets. When you put your penny into the slot it dropped into the mechanism and allowed the handle to open the lock.

The coin made a distinct click as it engaged in the internal workings, and hence the saying "the penny's dropped" meaning that the machine (your brain) will now operate.

[Chinajan]

Is that true? Actually, I thought the origin of this phrase came from the custom of placing pennies on the eyes of a deceased person to keep them closed. Therefore, if the "penny dropped" the eyes would open, indicating sudden awareness.

[xyzzyplugh]

agree with hippy, but vending machines.